USMLE Step 1 Qs (1)

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USMLE Step 1 Qs (1) - Quiz

For those of you who love all things biology and living organisms especially anatomy and how features work in the human body then this is the quiz for you. If you are prepared to test your knowledge, try it out.


Questions and Answers
  • 1. 

    . The secondary oocyte enters the second meiotic division and proceeds as far as metaphase. Which of the following is the stimulus required for continuation of the second meiotic division to produce the haploid ovum?

    • A.

      Elevation of progesterone titers

    • B.

      The environment of the oviduct and uterus

    • C.

      Expulsion from the mature follicle

    • D.

      Fertilization by a spermatozoon

    • E.

      The presence of human chorionic gonadotropin (hCG)

    Correct Answer
    D. Fertilization by a spermatozoon
    Explanation
    The secondary oocyte enters the second meiotic division just before ovulation and arrests at metaphase. Fertilization by a spermatozoon provides the stimulation for the division of chromatin to the haploid number. By the time the fertilized ovum reaches the uterus, the progesterone produced by the corpus luteum has initiated the secretory phase in the endometrium. Once implantation occurs and the chorion develops, human chorionic gonadotropin (hCG) is synthesized and the corpus luteum is maintained.

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  • 2. 

    The structures shown in the accompanying photomicrograph are derived from which of the following?

    • A.

      A combination of fetal and maternal tissues

    • B.

      Endometrial glands

    • C.

      Endometrial stroma

    • D.

      Fetal tissues

    • E.

      Maternal blood vessels

    Correct Answer
    D. Fetal tissues
    Explanation
    The placental structures shown in the photomicrograph are chorionic villi that are fetal tissues. The mother's contribution to the placenta is the blood that flows past the chorionic villi. A fertilized ovum reaches the uterus about four days after fertilization. At that time, it has developed into a multicellular, hollow sphere referred to as a blastocyst. The blastocyst soon adheres to the secretory endometrium and differentiates into an inner cell mass that will develop into the embryo and a layer of primitive trophoblast. The expanding trophoblast penetrates the surface endometrium and erodes into maternal blood vessels. Eventually, it develops two layers, an inner cytotrophoblast and an outer syncytiotrophoblast. Solid cords of trophoblast form the chorionic villi, which then are invaded by fetal blood vessels.

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  • 3. 

    Which of the following is responsible for the prevention of polyspermy, the fertilization of an oocyte by more than one sperm?

    • A.

      Resumption of the first meiotic division

    • B.

      Resumption of the second meiotic division

    • C.

      Capacitation

    • D.

      The zona reaction

    • E.

      The release of enzymes from the sperm acrosome

    Correct Answer
    D. The zona reaction
    Explanation
    On fusion of the first sperm with the oocyte cell membrane, the contents of secretory granules stored just beneath the oocyte membrane (cortical granules) are released (the zona reaction). Enzymes stored in these granules cause biochemical and electrical changes in the zona pellucida and the oocyte membrane that prevent the binding of additional sperm. Primitive female germ cells (oogonia) enter the first meiotic division during fetal development. This process becomes arrested in the metaphase stage until individual primary oocytes are hormonally induced to resume the first meiotic division during puberty and early adulthood (menarche to menopause). Fusion of the sperm and oocyte membranes initiates the resumption of the second meiotic division, resulting in the formation of a haploid pronucleus in the oocyte and extrusion of the second polar body. Capacitation is a process by which enzymatic secretions of the uterus and oviducts strip glycoproteins from the sperm cell membrane. This is required for penetration of the layer of cells surrounding the oocyte (corona radiata). The release of enzymes from the sperm acrosomal cap (an enlarged lysosome) results in digestion of the zona pellucida surrounding the oocyte, allowing penetration by sperm.

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  • 4. 

    Oogonia reach their maximum number at which of the following stages of human development?

    • A.

      Five months of fetal life

    • B.

      Birth

    • C.

      Puberty (12 to 14 years of age)

    • D.

      Adolescence (16 to 20 years of age)

    • E.

      Early adulthood (21 to 26 years of age)

    Correct Answer
    A. Five months of fetal life
    Explanation
    Maximum number of oogonia occurs at about the fifth month of development. Differentiation into oogonia begins once the primordial germ cells have arrived in the gonad of a genetic female. After undergoing a number of mitotic divisions, these fetal cells form a cluster in the cortical part of the ovary. Some of these oogonia differentiate into the larger primary oocytes, which by the third month of development are found in the deeper layers of the gonad. The primary oocytes begin meiosis to produce secondary oocytes. At the same time, the number of oogonia continues to increase to about 6,000,000 in the fifth month. At this time, most of the surviving oogonia and some of the oocytes become atretic. However, the surviving secondary oocytes (400,000 to 1,000,000) become surrounded by epithelial cells and form the primordial follicles by the seventh month. During childhood there is continued atresia so that by puberty only about 40,000 secondary oocytes remain.

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  • 5. 

    Blood from the placenta is about 80% oxygenated. However, mixture with unoxygenated blood at various points reduces the oxygen content. Which of the following vessels contains blood with the highest oxygen content?

    • A.

      Abdominal aorta

    • B.

      Common carotid arteries

    • C.

      Ductus arteriosus

    • D.

      Pulmonary artery

    • E.

      Pulmonary vein

    Correct Answer
    B. Common carotid arteries
    Explanation
    Blood from the placenta in the umbilical cord is about 80% oxygenated. Mixture with unoxygenated blood from the vitelline veins and the inferior vena cava reduces the oxygen content somewhat. However, this stream with a relatively high oxygen content is directed by the valve of the inferior vena cava directly through the foramen ovale into the left atrium. This prevents admixture with oxygen-depleted blood entering the right atrium from the superior vena cava. Thus, the oxygen-saturated blood entering the left ventricle and pumped into the aortic arch, subclavian arteries, and common carotid arteries has the highest oxygen content. The oxygen-depleted blood from the superior vena cava is directed into the right ventricle and then to the pulmonary trunk. Although a small portion of this flow passes through the lungs (where any residual oxygen is extracted by the tissue of the nonrespiring lung), most is shunted into the thoracic aorta via the ductus arteriosus and thereby lowers the oxygen content of that vessel. This occurs distal to the origins of the carotid arteries and ensures that the rapidly developing brain has the best oxygen supply.

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  • 6. 

    Which of the following hematopoietic tissues or organs develops from endoderm?

    • A.

      Thymus

    • B.

      Tonsils

    • C.

      Bone marrow

    • D.

      Spleen

    • E.

      Blood islands

    Correct Answer
    A. Thymus
    Explanation
    The thymic parenchyma (epithelial cells) develop from endoderm of the third pharyngeal (branchial) pouches. The thymic rudiment is invaded by bone marrow–derived lymphocyte precursors early in the third month of development. The tonsils develop as partially encapsulated lymph nodules. Their parenchymal framework is derived from pharyngeal mesoderm. Bones, of course, whether formed by intramembranous or endochondral ossification, are derived from mesoderm. Their forming marrow cavities are populated by hematopoietic stem cells beginning in the second month of fetal life. The connective tissue capsule and skeletal framework of the spleen develop from splanchnic lateral plate mesoderm during the fifth week and are quickly invaded by hematopoietic cells of the myeloid lineage. It remains an active hematopoietic organ until at least the seventh month in utero. Blood islands develop by differentiation of mesodermal cells in the extraembryonic mesoderm lining the yolk sac during the third week of fetal development. They give rise to vitelline vessels and are the major site of red blood cell formation in the early embryo.

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  • 7. 

    Which of the following processes is responsible for fusion of the paired dorsal aortae?

    • A.

      Lateral folding

    • B.

      Craniocaudal folding

    • C.

      Gastrulation

    • D.

      Neurulation

    • E.

      Looping of the heart tube

    Correct Answer
    A. Lateral folding
    Explanation
    The fusion of the dorsal aortae occurs through lateral folding. Craniocaudal folding establishes the definitive head and tail regions of the embryo. Fusion is already complete at the time that looping of the heart tube occurs. Fusion of the endocardial heart tube and incorporation of the yolk sac into the primitive gut also occurs as a result of lateral folding. Gastrulation establishes the three germ layers (trilaminar disk), and neurulation establishes the neural groove with two neural folds.

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  • 8. 

    The region labeled with the arrow in the accompanying electron micrograph of the plasma membrane is responsible for which of the following functions?

    • A.

      Creation of a barrier to water-soluble molecules

    • B.

      Specific cellular receptors for ligands

    • C.

      Catalyzing membrane-associated activities

    • D.

      Transport of small ions

    • E.

      Connections to the cytoskeleton

    Correct Answer
    A. Creation of a barrier to water-soluble molecules
    Explanation
    The hydrophobic layer of the cell (plasma) membrane is labeled with the arrow. It is responsible for the basic structure of the membrane and provides the barrier to water-soluble molecules in the external milieu. It also provides a two-dimensional solvent for membrane proteins. Other membrane functions are performed primarily by proteins that function as receptors, enzymes (catalysis of membrane-associated activities), and transporters. Connection to the cytoskeleton is performed by members of the spectrin family of proteins. The membrane consists of a bilayer of phospholipids with the nonpolar, hydrophobic layer in the central portion of the membrane and the hydrophilic polar regions of the phospholipids in contact with the aqueous components at the intra- or extracellular surfaces of the membrane. Proteins are generally dispersed within the lipid bilayer. The polar head groups of the lipid bilayer react with osmium to create the trilaminar appearance observed in electron micrographs of the plasma membrane. Cell membranes range in thickness from 7 to 10 nm [1 nm = 10–9 m, 1 m = 10–6 m; the diameter of a red blood cell (erythrocyte) is 7 m.

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  • 9. 

    Which of the following increases membrane fluidity under normal conditions?

    • A.

      Restriction of rotational movement of proteins and lipids in the membrane

    • B.

      Binding of integral membrane proteins with cytoskeletal elements

    • C.

      Transbilayer movement of phospholipids in the plasma membrane

    • D.

      High cholesterol content of the plasma membrane

    • E.

      Binding of an antibody to a cell surface receptor

    Correct Answer
    E. Binding of an antibody to a cell surface receptor
    Explanation
    Binding of an antibody to a cell surface receptor results in lateral diffusion of protein in the lipid bilayer, resulting in patching and capping. When such a divalent or multivalent ligand binds to membrane receptors present as intrinsic membrane proteins, initially there is a homogeneous pattern to the binding. Subsequently, the ligand-receptor complexes undergo patching and eventually capping on the cell surface. Rotational and lateral movements of both proteins and lipids contribute to membrane fluidity. Restriction reduces membrane fluidity. Phospholipids are capable of lateral diffusion, rapid rotation around their long axis, and flexion of their hydrocarbon (fatty acyl) tails. They undergo transbilayer movement, known as "flip-flop," between bilayers in the endoplasmic reticulum; however, this is a very rare occurrence in the plasma membrane. Other factors reduce membrane fluidity. Cholesterol at high concentrations decreases membrane fluidity by interacting with the hydrophobic regions near the polar head groups and stiffening this region of the membrane. Association or binding of integral membrane proteins with cytoskeletal elements on the interior of the cell and peripheral membrane proteins on the extracellular surface limit membrane mobility and fluidity.

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  • 10. 

    Members of the multiple-pass, G protein–linked family of receptors include the muscarinic, cholinergic, and -adrenergic receptors. Which of the following characterizes these receptors?

    • A.

      They possess intrinsic enzyme activity

    • B.

      They possess an arrangement of hydrophobic membrane-spanning segments

    • C.

      They possess an intracellular ligand-binding domain

    • D.

      They possess a single hydrophobic transmembrane segment

    • E.

      They are arranged so that both the amino- and carboxy-terminals are located intracellularly

    Correct Answer
    B. They possess an arrangement of hydrophobic membrane-spanning segments
    Explanation
    These receptors are all multipass transmembrane proteins consisting specifically of seven hydrophobic spanning segments of the single polypeptide chain. There is a remarkable homology between the cell surface receptors linked to the G proteins. Included in this group are the muscarinic and cholinergic receptors, rhodopsin, and the -adrenergic receptor. Between the segments, the polypeptide chain loops on both the extracellular and intracellular sides of the membrane. All of these transmembrane proteins show a carboxyl terminus on the cytosolic side and N-linked glycosylation sites on the extracellular surface. Receptors with intrinsic enzyme activity belong to a separate class of single-pass transmembrane proteins. Ligand binding occurs on the extracellular surface.

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  • 11. 

    Which of the following is the primary function of intermediate filaments?

    • A.

      Generate movement

    • B.

      Provide mechanical stability

    • C.

      Carry out nucleation of microtubules

    • D.

      Stabilize microtubules against disassembly

    • E.

      Transport organelles within the cell

    Correct Answer
    B. Provide mechanical stability
    Explanation
    There are differences in the way that intermediate filaments interact with microtubules and microfilaments within the cytoplasm; however, their ropelike arrangement is well suited to providing mechanical stability to the cell and resisting stretch, allowing the cell to respond to tension. The different types of intermediate filaments all have a similar structural pattern: nonhelical head and tail segments with a helical arrangement in the center of the intermediate filament structure. Movement is generated by motor proteins such as myosin, dynein, and kinesin. There is a good mnemonic device for remembering the direction of movement directed by kinesin and dynein. Kinesin kicks the molecules out; dynein drags them in; also, the plus end of the microtubule is oriented toward the plasma membrane, so minus end toward nucleus. This works for fibroblasts as well as neurons, to be discussed in a later chapter. Nucleation of microtubules is conducted by centrosomes. Microtubule-associated proteins (MAPs) stabilize or destabilize microtubules. Microtubules function in organellar transport (e.g., axonal transport).

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  • 12. 

    Which of the following mechanisms is used to establish the mitochondrial electrochemical gradient?

    • A.

      The action of ATP synthase

    • B.

      Pumping of protons into the mitochondrial matrix by respiratory chain activity

    • C.

      Transport of ATP out of the matrix compartment by a specific transporter

    • D.

      Proton-translocating activity in the inner membrane

    • E.

      Transfer of electrons from NADH to O2 in the intermembrane space

    Correct Answer
    D. Proton-translocating activity in the inner membrane
    Explanation
    The mitochondrial electrochemical gradient is established by a proton pump. The pump is located in the inner membrane, associated with the respiratory chain and ATP synthase. The impermeability of the inner membrane to protons causes an osmotic and electrochemical gradient to develop. Mitochondria produce energy that the cell uses in transport and other energy-dependent processes. Cellular energy is stored as ATP, which is synthesized by the phosphorylation of ADP by ATP synthase. Mitochondria use the electron-transport (respiratory) chain that transfers energy from NADH to O2. As electrons released by oxidation of substrate in the matrix flow down the respiratory chain, hydrogen ions are pumped into the intermembrane space. Protons in the matrix drive ATP synthase in a mechanism similar to that of a water wheel. ATP synthase, therefore, couples oxidative transport through the electron-transport (respiratory) chain with energy storage (ATP).

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  • 13. 

    Chloroquine is a weak base that neutralizes acidic organelles. In a pancreatic beta cell, which of the following would be a direct effect of chloroquine treatment?

    • A.

      Increased proinsulin content in secretory vesicles

    • B.

      Increased release of C peptide

    • C.

      Increased number of amylase-containing secretory vesicles

    • D.

      Reduced translation of glucagon mRNA

    • E.

      Increased stability of insulin mRNA

    Correct Answer
    A. Increased proinsulin content in secretory vesicles
    Explanation
    Chloroquine treatment inhibits the conversion of proinsulin to insulin, resulting in decreased formation of insulin within secretory vesicles. Chloroquine neutralizes acidic compartments such as the secretory vesicles. Acidification causes concentration of the contents of secretory vesicles, facilitates breakdown of the contents of phagosomes and lysosomes, and is involved in the cleavage of prohormones to their active forms (e.g., proinsulin to insulin). The acidification process functions through a vacuolar H+-proton pump that is present in the membranes of most endocytic and exocytic vesicles, including those of the phagosomes, lysosomes, secretory vesicles, and some compartments of the Golgi. Ribosomes are not dependent on a proton pump mechanism and are, therefore, less sensitive to chloroquine. Gene and message expression and message stability are also not targets for chloroquine. Note that pancreatic beta ( ) cells synthesize insulin. Proinsulin is split into C peptide + insulin in secretory vesicles. In vivo, C-peptide release can be used to measure production of insulin by a patient's pancreatic beta cells. This is particularly useful in patients who are receiving insulin. Glucagon is synthesized by alpha ( ) cells, and amylase is an exocrine pancreatic product produced by the acinar cells.

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  • 14. 

    Which of the following is the function of the nucleosomes?

    • A.

      Package genetic material in a condensed form

    • B.

      Transcribe the DNA

    • C.

      Form pores for bilateral nuclear-to-cytoplasmic transport

    • D.

      Form the nuclear matrix

    • E.

      Hold together adjacent chromatids

    Correct Answer
    A. Package genetic material in a condensed form
    Explanation
    Nucleosomes are the basic structural packaging unit of chromatin. Chromatin strands that have been treated to unpack the chromatin structure have the appearance of beads on a string in electron micrographs. The beads are formed by a core of histones as an octamer (i.e., two of each of the four nucleosomal histones: H2A, H2B, H3, and H4) plus two turns of DNA. The nucleosome beads plus the DNA between beads (i.e., linker DNA) constitute the nucleosome. There are additional orders of chromosome packing, including nucleosomal packing. The transcription of DNA is carried out by RNA polymerases I, II, and III, which are responsible for transcription of different types of genes. The nuclear pores are perforations in the nuclear envelope, each composed of a nuclear pore complex. The nuclear matrix is the intranuclear cytoskeleton and forms the scaffolding for nuclear structures. Chromatids are held together at the centromere.

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  • 15. 

    Which of the following is the primary function of the structure labeled between the arrows in the photomicrograph?

    • A.

      Extensive movement of substances over cell surfaces

    • B.

      Increase in surface area for absorption

    • C.

      Cell motility

    • D.

      Transport of intracellular organelles through the cytoplasm

    • E.

      Stretch

    Correct Answer
    B. Increase in surface area for absorption
    Explanation
    The structure labeled with the arrows in the figure is the brush border, also known as microvilli, which increases surface area for absorption. Microvilli are apical specializations of the epithelia. They are relatively uniform in length and are located on the luminal surface of cells such as small intestinal enterocytes, which are specialized for absorption. Microvilli are supported by a core of microfilaments and are capable of movement; however, cilia function in the movement of substances, such as mucus and foreign material, over the surface. The increased surface area of microvilli facilitates specialized uptake of molecules, which occurs by the processes of pinocytosis, receptor-mediated endocytosis, and phagocytosis, depending on the size and solubility of the molecules. Microtubules facilitate organellar movement within the cytoplasm, whereas cell movement is controlled by interactions between the cytoskeleton and the extracellular matrix. Transitional epithelium characteristic of the urinary system facilitates stretch.

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  • 16. 

    Which of the following is a function of the basement membrane?

    • A.

      Molecular filtering

    • B.

      Contractility

    • C.

      Excitability

    • D.

      Modification of secreted protein

    • E.

      Active ion transport

    Correct Answer
    A. Molecular filtering
    Explanation
    Epithelial cells require a basement membrane as a structural support. In most epithelia the basement membrane prevents penetration from the underlying lamina propria into the epithelium. Basement membranes are a pathway for migrating cells during development and repair processes (e.g., healing of skin wounds). In the kidney, the basement membrane of the renal glomerulus forms a selective barrier for the filtration of the plasma. Active ion transport and modification of secretory proteins are characteristics of the epithelia that are positioned on the basement membrane, not of the basement membrane itself. Contractility and excitability are characteristics that are not associated with basement membranes.

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  • 17. 

    Which of the following types of epithelium typically line the urinary system?

    • A.

      Simple columnar epithelium

    • B.

      Stratified squamous epithelium

    • C.

      Transitional epithelium

    • D.

      Pseudostratified ciliated epithelium

    • E.

      Simple squamous epithelium

    Correct Answer
    C. Transitional epithelium
    Explanation
    Transitional epithelium is associated with the urinary system including the lining of the renal calyces, ureters, and the bladder, as well as specific portions of the urethrae. The cells of the transitional epithelium increase in size toward the surface, or lumen. This is in contrast to the progressively flattened cells of the stratified squamous epithelium. The cells of the transitional epithelium vary in shape, depending on the degree of stretch of the wall. Epithelia perform a multitude of functions. Simple columnar epithelium is involved in absorption or secretion or both and forms most of the lining of the digestive tract. The stratified squamous epithelium is composed of several layers of cells, which begin with a basal layer of germinative cells where new cells are born and become progressively flattened toward the surface, or lumen. Stratified squamous epithelium provides a protective function and is found in sites such as the skin, where it is keratinized, and in the esophagus and anus, where it is nonkeratinized. The pseudostratified ciliated epithelium is located in the trachea and throughout the respiratory system and in the male reproductive system. As the name implies, the pseudostratified epithelium is not actually stratified. Nuclei at different levels present the appearance of stratification, but all cells reach the basal lamina. A simple squamous epithelium lines blood vessels (endothelium) and mesenteries (mesothelium), and its structure facilitates transport functions. Stratified cuboidal epithelium and stratified columnar epithelium line the sweat ducts and the excretory ducts of the parotid gland, respectively.

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  • 18. 

    The extracellular matrix and the cytoskeleton communicate across the cell membrane through which of the following structures?

    • A.

      Proteoglycans

    • B.

      Integrins

    • C.

      Cadherins

    • D.

      Intermediate filaments

    • E.

      Microtubules

    Correct Answer
    B. Integrins
    Explanation
    The integrins are transmembrane heterodimers that act as membrane receptors for extracellular matrix components. The best examples are the fibronectin receptor and the laminin receptor. The receptor structure includes an intracytosolic portion that binds to the actin cytoskeleton through the attachment proteins talin or -actinin. The extracellular portion has specificity for extracellular matrix molecules. The N-cadherins function as transmembrane glycoproteins involved in the formation of parts of the intercellular junctional complexes. Proteoglycans are located on the extracellular surface of the plasma membrane and throughout the extracellular matrix. Cadherins are components of the desmosome and zonula adherens. Microtubules and intermediate filaments are found intracellularly and constitute the cytoskeleton.

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  • 19. 

    Which of the following is the function of fibronectin in the extracellular matrix?

    • A.

      Structural support

    • B.

      Binding of signaling molecules

    • C.

      Selectivity for passage of molecules

    • D.

      Elasticity

    • E.

      Adhesion and cell attachment

    Correct Answer
    E. Adhesion and cell attachment
    Explanation
    Fibronectin is an adhesive glycoprotein that is important for cell attachment. It is important for modulation of cell migration in the adult and during development. Neural crest and other cells appear to be guided along fibronectin-coated pathways in the embryo. Fibronectin is found in three forms: a plasma form that is involved in blood clotting; a cell surface form, which binds to the cell surface transiently; and a matrix form, which is fibrillar in arrangement. Fibronectin contains a cell-binding domain (RGD sequence), a collagen-binding domain, and a heparin-binding domain. Type IV collagen is responsible for providing support. Elastin is responsible for the elasticity of structures such as the pinna of the ear and the wall of the aorta. Proteoglycans are responsible for the selective passage of molecules and the binding of growth factors and other signaling molecules in the extracellular matrix.

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  • 20. 

    Ehlers-Danlos syndrome occurs in several forms. In type IV disease there is a defect in type III collagen synthesis. Which of the following symptoms would be most expected in a patient with this disorder?

    • A.

      Rupture of the intestinal or aortic walls

    • B.

      Hyperextensibility of the integument

    • C.

      Hypermobility of synovial joints

    • D.

      Increased degradation of proteoglycans in articular cartilages

    • E.

      Imperfections in dentin formation (dentinogenesis imperfecta)

    Correct Answer
    A. Rupture of the intestinal or aortic walls
    Explanation
    In Ehlers-Danlos disorder type IV there is improperly formed type III collagen, which is responsible for the elasticity of the intestinal and aortic walls. In this form of the disorder there are errors in the transcription of type III collagen mRNA or in translation of this mRNA. Hyperextensible skin occurs in Ehlers-Danlos type VI disorder in which problems with the hydroxylation of the amino acid lysine and subsequent cross-linking result in enhanced elasticity. Type VII Ehlers-Danlos disorder involves a specific deficiency in an amino terminal procollagen peptidase. This results from a genetic mutation that alters the propeptide sequence in such a way that the molecular orientation and cross-linking are adversely affected. The result is hypermobility of synovial joints. Increased degradation of proteoglycans occurs in osteoarthritis. Type I collagen is found in dentin.

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  • 21. 

    . Which of the following is the primary function of brown adipose tissue?

    • A.

      To store unilocular energy

    • B.

      To provide thermal insulation

    • C.

      To mobilize lipid for export as fatty acids

    • D.

      To initiate the shivering-induced mobilization of lipid

    • E.

      To produce heat

    Correct Answer
    E. To produce heat
    Explanation
    Brown adipose tissue is multilocular and is found in the human fetus and neonate. Brown fat is involved in nonshivering thermogenesis and generates heat, probably as a protective device for developing organs in the fetus and neonate. Adipose tissue is specialized for lipid storage and functions as a thermal insulator and shock absorber. White adipose tissue is unilocular, and the cells have a single, large lipid droplet in the cytoplasm that provides the "signet-ring" appearance often described for fat cells. Brown adipose tissue has a multilocular appearance and a brown color because of the many mitochondria in these adipocytes. Both types of fat tissue are highly vascularized and function in protection from the cold. Brown fat specifically is involved in heat production, whereas white fat is a true thermal insulator. The former is found in hibernating animals and neonatal humans. In fat, norepinephrine activates the cyclic AMP cascade through adenylate cyclase. The cyclic AMP activates hormone-sensitive lipase, which removes triglycerides from the stored lipid and hydrolyzes free fatty acids. In white adipocytes, the released fatty acids and glycerol are exported from the cells. In brown adipose tissue the fatty acids are used within the cell. However, the electron transport system is uncoupled from oxidative phosphorylation, which results in the production of heat instead of ATP. Heat is transferred to the blood by the extensive capillary networks found in brown adipose tissue.
    Shivering initiates the mobilization of lipid in white adipose tissue because shivering requires energy.

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  • 22. 

    Which of the following is a major contributor to the tensile strength of collagen?

    • A.

      Interactions with the FACIT collagens

    • B.

      The double helical arrangement of collagen

    • C.

      Electrostatic interactions

    • D.

      Intramolecular and intermolecular cross-links

    • E.

      Low concentrations of lysine

    Correct Answer
    D. Intramolecular and intermolecular cross-links
    Explanation
    The fibrillar collagens establish tensile strength at a number of levels including intra- and intermolecular cross-links. Covalent bonding occurs through the OH– groups of hydroxylysine and hydroxyproline and serves to stabilize the triple helix. The triple helix itself functions to resist tensile forces. The degree of cross-linking varies from tissue to tissue. For example, it is highly extensive in tendons. The organization of collagen in tissues also varies, depending on function, from the layered appearance in bone to the axial parallel bundles in tendons and the wickered pattern in skin. The interactions with fibril-associated collagens (with interrupted triple helices) regulate orientation and are also important in establishing tissue organization and flexibility. Electrostatic interactions do not play a significant role in maintenance of collagen tensile strength.

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  • 23. 

    A patient with rheumatoid arthritis would exhibit which of the following joint changes?

    • A.

      Loss of the proteoglycan matrix and fibrillation in the articular cartilage during the early stages

    • B.

      Decreased levels of fibrinogen in the synovial fluid

    • C.

      Formation of osteophytes at the articular margins and eburnation of large weight-bearing joints in the later stages

    • D.

      Decreased number of leukocytes including PMNs in the synovial fluid

    • E.

      Heterologous autoantibodies deposited in joint surface

    Correct Answer
    E. Heterologous autoantibodies deposited in joint surface
    Explanation
    Arthritis involves inflammatory changes in a joint. Rheumatoid arthritis is an autoimmune disease in which a rheumatoid factor composed of heterologous autoantibodies directed against serum gamma-globulin (IgG) appears. Rheumatoid factor is present in the serum of 85 to 90% of patients with rheumatoid arthritis. Deposition of rheumatoid factor can be pathogenic and leads to inflammatory destruction of the joint surface. Cell-mediated immunity is also involved in rheumatoid arthritis. Alteration of the synovial membrane results in the formation of a pannus, or inflammatory, hypertrophic synovial villus. The presence of the pannus and release of lysosomal enzymes from the pannus result in degradation of the cartilage. This is followed by hypertrophy and hyperplasia of the articular cartilages, which often leads to bone formation across the joint with welding of the bones together (ankylosis). Because of the inflammation in rheumatoid arthritis, there are elevated numbers of leukocytes, in particular PMNs, in the synovial fluid. During rheumatoid arthritis, fibrinogen, another indicator of inflammatory responses, is elevated. Osteoarthritis begins with loss of hydrated glycosaminoglycans, followed by death of chondrocytes, fibrillation, and development of fissures in the cartilage matrix. The severe wear and tear of osteoarthritis increases with age. During the breakdown of the articular cartilages, the width of the underlying bone increases. Osteoarthritis typically includes the formation of reactive bone spurs called osteophytes, which may break off to form foreign bodies in the joint space (i.e., "joint mice"). In the fingers, osteoarthritis primarily affects distal interphalangeal joints, where it produces painful nodular enlargements called Heberden's nodes. Large weight-bearing joints are also usually involved in osteoarthritis and often exhibit eburnation in the late stages when the articular cartilages have been worn down and result in an osseous articular surface

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  • 24. 

    Patients with Cushing's syndrome often show osteoporotic changes. Which of the following is involved in the etiology of osteoporosis induced by Cushing's syndrome?

    • A.

      Decreased glucocorticoid levels that result in decreased quality of the bone deposited

    • B.

      Excess deposition of osteoid

    • C.

      Stimulation of intestinal calcium absorption

    • D.

      Decreased PTH levels

    • E.

      Bone fragility resulting from excess bone resorption

    Correct Answer
    E. Bone fragility resulting from excess bone resorption
    Explanation
    Osteoporosis is a major problem of normal aging in both sexes but is particularly prevalent in older women. In this disease, the quality of bone is unchanged, but the balance between bone deposition and bone resorption is lost. The disease is prevalent in postmenopausal women because the protective effect of estrogens is no longer present. Osteoporosis may also be induced by other diseases (e.g., hyperthyroidism) or drugs (e.g., alcohol and caffeine). In addition, excess glucocorticoids induce osteoporosis. For example, in Cushing's syndrome, patients produce high levels of corticosteroids that interfere with bone metabolism. A similar pattern may be seen during prolonged steroid therapy. The result is increased bone resorption compared with bone deposition. Intestinal calcium absorption is inhibited and PTH levels may be increased.

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  • 25. 

    The collagenous protein in bone subserves which of the following functions?

    • A.

      Growth factor

    • B.

      Binding of ionic calcium and physiologic hydroxyapatite

    • C.

      Formation of the three-dimensional lattice of the matrix

    • D.

      Cell attachment

    • E.

      Binding of mineral components to the matrix

    Correct Answer
    C. Formation of the three-dimensional lattice of the matrix
    Explanation
    Type I collagen is responsible for the three-dimensional fiber structure of the matrix. It is synthesized by osteoblasts and accounts for 85 to 90% of total bone protein. The noncollagenous bone proteins are primarily synthesized by osteoblasts and constitute 10 to 15% of bone protein. Some plasma proteins are preferentially absorbed by the bone matrix. The noncollagenous proteins include cytokines and growth factors, which are synthesized endogenously and become trapped in the matrix. Also included in the category of noncollagenous proteins are the cell attachment proteins (fibronectin and osteopontin); proteoglycans (e.g., chondroitin 4-sulfate and chondroitin 6-sulfate), which appear to play a role in collagen fibrillogenesis; and the gla proteins, such as osteocalcin (containing -carboxyglutamic acid), which binds Ca2+ and mineral components to the matrix.

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  • 26. 

    In a given muscle fiber at rest, the length of the I band is 1.0 mm and the A band is 1.5 mm. Which of the following is the length of the sarcomere?

    • A.

      4.0 micrometer

    • B.

      3.5 micrometer

    • C.

      2.5 micrometer

    • D.

      2.0 micrometer

    • E.

      1.5 micrometer

    Correct Answer
    C. 2.5 micrometer
    Explanation
    During contraction, the sarcomere, the distance between adjacent Z lines, decreases in length, and the length of the A band is almost constant. However, as the degree of overlap of thick and thin filaments is altered, the thin filaments, which form the I band and are anchored to the Z line, are pulled toward the center of the sarcomere. As this occurs, the I band decreases in length and the H band is no longer visible. The filaments themselves do not decrease in length; they slide past one another in the sliding-filament model of muscle contraction.

    The average length of a sarcomere is 2.5Um. This distance is measured from one Z line to the next Z line. If the resting length of the A band is 1.5 Um and the length of the I band is 1.0 Um, then the resting length of the sarcomere is determined by adding the length of the I band to the length of the A band. If there is a 20% contraction of the muscle (contraction to 80% of its length), then the sarcomere is reduced in length from 2.5 to 2.0 Um. The size of the A band remains unchanged (whether the contraction is 10 or 20%); therefore, the length of the I band is reduced from 1.0 to 0.5 Um and makes up for the 0.5-Um reduction in length during muscle contraction.

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  • 27. 

    The actin-rich cell cortex is involved in which of the following cell functions?

    • A.

      Cytokinesis

    • B.

      Chromosomal movements

    • C.

      Bidirectional transport of vesicles

    • D.

      Fast axoplasmic transport

    • E.

      Ciliary movement

    Correct Answer
    A. Cytokinesis
    Explanation
    The cell cortex is an area of the cell immediately underneath the plasma membrane and is rich in actin, which is required for cytokinesis. This region is important in maintaining the mechanical strength of the cytoplasm of the cell. It is also essential for cellular functions that require surface motility. These functions include phagocytosis, cytokinesis, and cell locomotion. Although movement of vesicles along filaments is regulated by minimyosins (myosin I), movement of vesicles and organelles is predominantly a function of microtubules under the influence of the unidirectional motors kinesin and dynein. The movements of cilia and flagella are driven by dynein and chromosomal movements occur through microtubular kinetics.

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  • 28. 

    Which of the following is absent from smooth muscle cells?

    • A.

      Troponin

    • B.

      Calmodulin

    • C.

      Calcium

    • D.

      Myosin light chain kinase

    • E.

      Actin and tropomyosin interactions similar to skeletal muscle

    Correct Answer
    A. Troponin
    Explanation
    Smooth muscle is the least specialized type of muscle and contains no troponin. The contractile process is similar to the actin-myosin interactions that occur in motility of nonmuscle cells. In the smooth muscle cell, actin and myosin are attached to intermediate filaments at dense bodies in the sarcolemma and cytoplasm. Dense bodies contain alpha-actinin and, therefore, resemble the Z lines of skeletal muscle. Contraction causes cell shortening and a change in shape from elongate to globular. Contraction occurs by a sliding filament action analogous to the mechanism used by thick and thin filaments in striated muscle. The connections to the plasma membrane allow all the smooth muscle cells in the same region to act as a functional unit. Sarcoplasmic reticulum is not as well developed as that in the striated muscles. There are no T tubules present; however, endocytic vesicles called caveolae are believed to function in a fashion similar to the T tubule system of skeletal muscle.

    When intracellular calcium levels increase, the calcium is bound to the calcium-binding protein calmodulin. Ca2+-calmodulin is required and is bound to myosin light chain kinase to form a Ca2+-calmodulin-kinase complex. This complex catalyzes the phosphorylation of one of the two myosin light chains on the myosin heads. This phosphorylation allows the binding of actin to myosin. A specific phosphatase dephosphorylates the myosin light chain, which returns the actin and myosin to the inactive, resting state. The actin-tropomyosin interactions are similar in smooth and skeletal muscle.

    Smooth muscle cells (e.g., vascular smooth muscle cells) also differ from skeletal muscle cells in that they are capable of collagen, elastin, and proteoglycan synthesis, which is usually associated with fibroblasts.

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  • 29. 

    Which of the following mechanochemical enzymes can be found on the surfaces of cellular organelles where it mediates movement toward the plus end of microtubules?

    • A.

      Myosin (myosin II)

    • B.

      Minimyosin (myosin I)

    • C.

      Dynein

    • D.

      Kinesin

    • E.

      Filamin

    Correct Answer
    D. Kinesin
    Explanation
    Kinesin moves vesicles unidirectionally from the minus end to the plus end of the microtubule, e.g., from the cell body to the axon terminus in fast axonal transport. Myosin, minimyosin, dynein, dynamin, and kinesin are all mechanochemical enzymes or molecular "motors" that hydrolyze ATP and undergo conformational changes that are converted into movement. Cytoplasmic dynein is responsible for movement toward the minus end of the microtubules. Remember kinesin kicks out and dynein drags them in (see feedback question 40). Ciliary and flagellar bending is the classic model for microtubule-based motility. The motor is dynein, which causes the relative sliding between microtubules in the axoneme. Structural constraints within the axoneme as a whole convert sliding into ciliary bending.

    Dynamin is another ATPase motor that mediates sliding between adjacent cytoplasmic microtubules. Filamin or other actin cross-linking proteins form a gel network in the cell cortex (the area just beneath the cell membrane). The presence of the actin gel in the cell cortex contributes to the rigidity of the cell and is also involved in changes in cell shape.

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  • 30. 

    In the accompanying photomicrograph, which of the following is the structure labeled C?

    • A.

      Perineurium

    • B.

      Epineurium

    • C.

      Endomysium

    • D.

      Myelin sheath

    • E.

      Tunica media

    Correct Answer
    A. Perineurium
    Explanation
    In this high–magnification light micrograph, several cross sections through small peripheral nerves are visible. "C" indicates the perineurium, a layer of two to three fibroblast-like cells with contractile properties that surround individual fascicles. Cells of the perineurium are joined by tight junctions and form a barrier to macromolecules. "B" indicates dense irregular connective tissue surrounding the small nerves. They contain numerous nerve fibers surrounded by myelin sheaths (D) produced by Schwann cells (nucleus visible at E). Other nuclei visible within the fascicle include (1) those of fibroblasts, which secrete the reticular connective tissue elements forming the endoneurium surrounding the individual neuronal fibers and (2) nuclei of capillary endothelial cells. Neuronal perikarya are not present in peripheral nerves. "A" indicates skeletal muscle, identifiable by its striations and peripherally located nuclei.

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  • 31. 

    The structure labeled C in the accompanying electron micrograph of a synapse is the site of which of the following?

    • A.

      Neurotransmitter uptake in synaptic vesicles by endocytosis

    • B.

      Postsynaptic receptors

    • C.

      Neurotransmitter-induced alteration of membrane permeability

    • D.

      Membrane continuity between adjacent neurons

    • E.

      Recycling of synaptic vesicle membranes by endocytosis

    Correct Answer
    E. Recycling of synaptic vesicle membranes by endocytosis
    Explanation
    The electron micrograph of the synapse shows the presynaptic membrane (C), postsynaptic membrane (D), mitochondria (B), and synaptic vesicles (A). Recycling of synaptic vesicle membrane occurs at the presynaptic membrane in conjunction with neurotransmitter release by exocytosis. Neurotransmitter release is induced by membrane depolarization, leading to transient opening of calcium channels followed by calcium influx. There is no cytoplasmic continuity between adjacent neurons. Transmission from neuron to neuron occurs by chemical transmission in the form of neurotransmitter release. The neurotransmitter (from the synaptic vesicles) crosses the synaptic cleft (between the pre- and postsynaptic membranes) and interacts with receptors on the postsynaptic membrane, which results in changes in the permeability of this membrane. Numerous mitochondria and synaptic vesicles are typically found on the presynaptic side of the synapse. The postsynaptic surface typically is more dense than the presynaptic membrane.

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  • 32. 

    In the human fetus, which of the following is the predominant site of the process shown in the figure below in gestational months 5 to 9?

    • A.

      Liver

    • B.

      Yolk sac

    • C.

      Spleen

    • D.

      Thymus

    • E.

      Bone marrow

    Correct Answer
    E. Bone marrow
    Explanation
    The first stage in granulopoiesis is the myeloblast (A), a large cell with prominent light-staining nucleoli with only a little cytoplasm, generally without granules. The lineage shown in the figure illustrates eosinophilic development in the bone marrow. The promyelocyte (B) is the next cell in the lineage. It is larger than the myeloblast, nucleoli are less visible, and primary granules are present in the cytoplasm. In the next stage, myelocyte-specific granules are seen in the cytoplasm with flattening of the nucleus. The eosinophilic myelocyte (C) differentiates into the eosinophilic metamyelocyte (D) when invagination of the nucleus begins. Further invagination leads to the formation of an eosinophilic band (E) and ultimately a mature eosinophil (F). An eosinophil has a bilobed nucleus and plays an important role in allergic and parasitic infections. The granules stain with eosinophil dyes and contain major basic protein, histaminase, peroxidase, and some hydroletic enzymes. This cell has an affinity for antigen-antibody complexes and, although phagocytic, it is not as active against bacteria as neutrophils. The histaminase secreted by eosinophils counteracts the release of histamine from basophils and mast cells, essential in hypersensitivity reactions. B lymphocytes differentiate into antibody-producing plasma cells; T lymphocytes are primarily responsible for graft rejection; and neutrophils are responsible for phagocytosis of bacteria.
    Hematopoiesis occurs in the flat bones of the skull and other bones in the adult human. Although most bones in the body are involved in hematopoiesis during growth, the marrow of the sternum, ribs, vertebrae, iliac crest, skull, and proximal femora are the primary sites of blood cell development by the time that skeletal maturity is achieved. It also occurs in the long bones during development, but many of these areas become dominated by yellow marrow that contains many fat cells (adipose tissue). The inactive yellow marrow can be reactivated on exposure to the proper stimulus (e.g., severe blood loss).
    During prenatal development the first site of blood cell development (hematopoiesis) is extraembryonic, in the yolk sac. The yolk sac produces hematocytoblasts and primitive erythroblasts from the third week through the second month of gestation. Hepatic erythropoiesis begins during the sixth week, reaches its maximum in the third month, and then ceases about the seventh month. The bone marrow begins to function in the second month and becomes the predominant hematopoietic site during months 5 to 9 of gestation, whereas the spleen is involved specifically in the production of red blood cells (erythropoiesis) from months 2 to 5 of gestation with some activity continuing until shortly after birth. Although erythropoiesis ceases in the spleen, this organ continues to produce monocytes and lymphocytes throughout life. In addition, from the second month the lymph nodes produce lymphocytes, and the thymus is responsible for the education of T cells after the second month of gestation. These T lymphocytes seed to T-dependent areas, such as the deep cortex of the lymph node.
    The kidney produces erythropoietin, a growth factor that stimulates red cell development, but it is not considered a site of blood cell development.

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  • 33. 

    Erythrocytes may have abnormal shapes and sizes in certain diseases. In iron deficiency you would expect to see which of the following?

    • A.

      Microcytic, hypochromatic anemia with smaller mature erythrocytes

    • B.

      Macrocytic, hyperchromatic anemia with fewer, larger mature erythrocytes

    • C.

      Poikilocytosis (shape change) and more fragile erythrocytes

    • D.

      Spherical rather than biconcave erythrocytes

    • E.

      No change in erythrocyte size or shape, but a substantial drop in the hematocrit

    Correct Answer
    A. Microcytic, hypochromatic anemia with smaller mature erythrocytes
    Explanation
    In iron deficiency, anemia results with the presence of smaller, pale-staining erythrocytes (microcytic, hypochromatic). In hemolytic anemia, there is excessive destruction of red blood cells in the spleen. Hyperchromic, macrocytic anemia results from vitamin B12 deficiency. The presence of spherical rather than biconcave erythrocytes is associated with spherocytosis, which often results in hemolysis. The membrane undergoes deformation due to the inability of ankyrin to bind spectrin.

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  • 34. 

    A 43-year-old woman who has suffered from diabetes for 30 years comes into the clinic. She is anemic with a hematocrit of 22. Which of the following would most likely explain her condition?

    • A.

      Decreased hepatic production of erythropoietin, leading to decreased numbers of circulating reticulocytes in the bloodstream

    • B.

      Increased erythropoietin production by the liver, resulting in increased numbers of reticulocytes

    • C.

      Decreased renal erythropoietin production, leading to reduced numbers of red blood cells

    • D.

      Decreased estrogen levels, stimulating hepatic production of erythropoietin

    • E.

      Decreased estrogen levels, inhibiting renal production of erythropoietin

    Correct Answer
    C. Decreased renal erythropoietin production, leading to reduced numbers of red blood cells
    Explanation
    The most likely cause of the anemia is renal failure, leading to decreased production of the kidney–derived red blood cell growth factor, erythropoietin. In the initial stages of renal failure the kidneys will increase their production of erythropoietin, but as renal damage continues the cells that produce this factor are destroyed. Therefore, there are initially increased levels of reticulocytes (immature red blood cells) in the bloodstream, but as in the anemia of renal disease, low production of reticulocytes is a hallmark of the disease. Although the patient may have decreased estrogen levels, estrogen decreases hematocrit. Also, women who are pregnant (third trimester) can have slightly decreased hematocrits [37 ± 6 (third trimester pregnant women) vs. 40 ± 6 (adult women) and 42 ± 6 (postmenopausal women)]. Administration of recombinant erythropoietin (EPO) is the preferred treatment for anemia caused by advanced renal disease. Generally, EPO is administered if the hematocrit is less than 30% and whether the patient is on dialysis.

    Erythropoietin is synthesized by the peritubular (interstitial) cells of the kidney cortex, stimulates the differentiation of cells from the erythrocyte colony–forming units (E-CFUs) and stimulates the differentiation and release of reticulocytes from the bone marrow. Colony-forming units (CFUs) are distinct cell lineages derived from pluripotential stem cells in the bone marrow.

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  • 35. 

    Which of the following is a metabolic function of endothelial cells?

    • A.

      Formation of angiotensinogen

    • B.

      Activation of bradykinin

    • C.

      Production of type III collagen

    • D.

      Synthesis of plasminogen activator

    • E.

      Production of thromboxane

    Correct Answer
    D. Synthesis of plasminogen activator
    Explanation
    Endothelial cells synthesize a number of antithrombogenic factors including plasminogen activator and prostacyclin. Prostacyclin functions through cyclic AMP to inhibit thromboxane production by platelets. Endothelial cells synthesize the basal lamina including types IV, V, and VIII collagens, fibronectin, and laminin. Secretion of A and B blood group antigens also occurs in endothelial cells. Angiotensin converting enzyme on the endothelial cell surface converts angiotensin I to angiotensin II (a potent vasoconstrictor), but also serves as an inactivation enzyme (bradykininase) for bradykinin, a vasodilator. The endothelium produces nitric oxide, also known as endothelium–derived relaxing factor (EDRF), and endothelin, the most potent vasoconstrictor in the body. Endothelial cells also synthesize plasminogen inhibitor, a coagulant and von Willebrand factor (factor VIII) which is found in Weibel-Palade granules in endothelial cells of vessels larger than capillaries. A deficiency of factor VIII leads to decreased platelet aggregation and hemophilia.

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  • 36. 

    Antigen-specific cell-mediated immunity, as occurs in graft rejection, results directly from the activity of which type of cell?

    • A.

      T lymphocytes

    • B.

      Plasma cells

    • C.

      Monocytes

    • D.

      Eosinophils

    • E.

      Mast cells

    Correct Answer
    A. T lymphocytes
    Explanation
    Transplant (graft) rejection is mediated by the action of T cells. Helper T (TH) cells recognize peptides associated with MHC antigens from the donor tissue and become activated. Activated TH cells release interleukin 2 (IL-2) and interferon-gamma (IFN-r ), which activate cytotoxic T (TC) cells, B cells, and macrophages. Although all of these cells are involved in the rejection process, T cells are the primary agents of transplant rejection. This has been confirmed by animal studies: animals deficient in T cells are incapable of rejecting grafts.

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  • 37. 

    Gene rearrangement of cytotoxic T cells occurs primarily in which of the following sites?

    • A.

      Bone marrow

    • B.

      Spleen

    • C.

      Germinal centers

    • D.

      Thymus

    • E.

      Mesenteric lymph nodes

    Correct Answer
    D. Thymus
    Explanation
    T cell gene rearrangement occurs during the education of T cells in the thymus in fetal and early neonatal development. The T cell receptor (TCR) is composed of and chains. Each chain contains a variable amino-terminal portion and a constant carboxyl-terminal portion. These chains are encoded for by V, D, J, and C gene segments, which undergo rearrangement during development in the thymus. There are three types of T cells: cytotoxic T cells, suppressor T cells, and helper T cells. Helper T cells possess a specific cell membrane marker and are known as CD4+ cells, whereas CD8+ cells have a different cell surface marker and include the suppressor and cytotoxic subgroups. T cells require close contact with other cells to perform their cell-mediated function. This is quite different from B cells, where antibodies are secreted into the bloodstream. B cell gene rearrangement occurs in the bone marrow during B cell education by a similar process. It must be remembered that T cell receptors are antibody-like heterodimers. Gene rearrangement in B and T cell education involves similar V(D)J recombinations.

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  • 38. 

    Gene products of class II major histocompatibility complex (MHC) present antigenic peptides primarily to which of the following cells?

    • A.

      Plasma cells

    • B.

      B cells

    • C.

      Antigen-presenting cells

    • D.

      Helper T cells

    • E.

      Cytotoxic T cells

    Correct Answer
    D. Helper T cells
    Explanation
    Fragments of antigen associated with class II MHC glycoproteins are recognized by helper T cells. Activation of helper T cells is required as an early step in the immune response. For B cells to respond to most antigens, helper T cells are an absolute requirement. However, in the case of some bacterial polysaccharides, B cells respond to the antigens in the absence of helper T cells. The primary cell type for expression of class II MHC is the macrophage that serves as an antigen-presenting cell, but thymic epithelial cells and B cells can also present antigen under appropriate conditions

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  • 39. 

    The tissue immediately surrounding the structure labeled with the arrow functions primarily as which of the following?

    • A.

      T cell-dependent area

    • B.

      B cell-dependent area

    • C.

      Region specialized for phagocytosis

    • D.

      Region specialized for T cell education

    • E.

      Region specialized for erythrocyte destruction

    Correct Answer
    A. T cell-dependent area
    Explanation
    The region shown is a T-dependent region of the spleen. The photomicrograph is a medium-magnification view of the spleen. The region shown is an area of white pulp with a central artery. The sheath surrounding the central artery is known as the periarterial lymphoid sheath (PALS) and is analogous to the deep cortex (paracortex) of the lymph node or the interfollicular zone of Peyer's patches, the other T-dependent regions within lymphoid tissue. The histologic structure of the spleen includes the presence of a connective tissue capsule with extensions into the parenchyma, forming trabeculae. The parenchyma consists of red pulp, which represents areas of red blood cells, many of which are undergoing degradation and phagocytosis by macrophages lining the sinusoids of the red pulp, and white pulp, which represents lymphocytes involved in the filtration of the blood. The germinal centers within the white pulp are the B-dependent regions of the spleen.

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  • 40. 

    The cell shown in the accompanying photomicrograph was treated with fluorescein-labeled antihuman immunoglobulin. This cell would most likely be found in which of the following sites?

    • A.

      Cortex of the thymus

    • B.

      Medulla of the thymus

    • C.

      Deep cortex (paracortex) of the lymph node

    • D.

      Germinal centers in the spleen

    • E.

      Periarterial lymphoid sheath in the white pulp of the spleen

    Correct Answer
    D. Germinal centers in the spleen
    Explanation
    Immunoglobulin-producing cells would be found in the germinal centers of the white pulp of the spleen and other secondary lymphoid organs (i.e., germinal centers in the cortex of the spleen and in the tonsils and lymphoid follicles in the MALT). Lymphocytes that are the equivalent of the bursa-dependent B lymphocytes of the chicken can be identified by the presence of immunoglobulin on their surface membranes. These are the cells that ultimately differentiate into antibody-secreting plasma cells under the appropriate conditions. T lymphocytes, on the other hand, do not have readily detectable cell membrane immunoglobulin. The thymus and the periarterial lymphoid sheath would contain T cells.

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  • 41. 

    Which of the following would occur during a viral infection?

    • A.

      Phagocytosis of virus by CD4+ T cells

    • B.

      Presentation of antigen by CD4+ T cells

    • C.

      Killing of virus-infected cells by CD4+ T cells

    • D.

      Formation of memory B and memory T cells

    • E.

      Killing of virus-infected cells by CD4+ T cells

    Correct Answer
    D. Formation of memory B and memory T cells
    Explanation
    During a viral infection, both cell-mediated and humoral responses are stimulated. Therefore, both memory T and memory B cells will be formed. B cells will divide to form a plasma cell and a memory B cell. Activated T cells also enlarge to form large lymphocytes and subsequently undergo cell proliferation to form T cells and memory T cells. In these responses, macrophages phagocytose virus. Cells that become infected with virus can be killed by CD8+ cytotoxic T cells, which can react to the antigen in the presence of MHC class I molecules. T and B cell areas of the spleen and lymph nodes will be involved in the filtration of the blood and lymph, respectively. B cell differentiation requires the presence of CD4+ helper T cells and an antigen-presenting cell. The antigen-presenting cell will phagocytose the virus and present it to helper T cells in the presence of MHC class II molecules. The B cell also presents antigen during viral infections.

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  • 42. 

    Macrophages are directly involved in immune responses in which of the following ways?

    • A.

      Production of IL-2

    • B.

      Presentation of antigen

    • C.

      Specific killing of tumor cells

    • D.

      Production of antibodies

    • E.

      Inactivation of helper T cells

    Correct Answer
    B. Presentation of antigen
    Explanation
    Macrophages are a group of monocyte-derived phagocytic cells that present antigen and synthesize IL-1. Macrophages arise from the bone marrow (monocytes) and include the Kupffer cells of the liver, Langerhans cells of the skin, and microglia of the central nervous system. Antigen presentation is the process by which macrophages and dendritic cells (antigen-presenting cells) phagocytose antigen and partially degrade the antigen in the endosomal system. Certain portions of the antigen are returned to the cell surface. IL-1 is a lymphokine (i.e., cytokine) that activates the helper T cell. Although in many cases macrophages are required for the differentiation of plasma cells from B cells, they are not directly involved in antibody production.

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  • 43. 

    A 52-year-old male patient, who has smoked two packs of cigarettes per day for the past 38 years, presents with diminished breath sounds detected by auscultation accompanied by faint high-pitched rhonchi at the end of each expiration and a hyperresonant percussion note. In addition, he shows discomfort during breathing and is using extra effort to involve accessory muscles to lift the sternum. The diminished lung sounds in this patient are primarily due to which of the following cellular events?

    • A.

      Monocytic infiltration leading to collagenase destruction of bronchiolar connective tissue support

    • B.

      Neutrophilic infiltration leading to destruction of bronchiolar and septal elastic fibers

    • C.

      Monocytic infiltration leading to breakdown of the bronchiolar smooth muscle

    • D.

      Neutrophilic infiltration leading to excess production of antiprotease activity in the lung parenchyma

    • E.

      Monocytic infiltration leading to excess production of antiprotease activity in the lung parenchyma

    Correct Answer
    B. Neutrophilic infiltration leading to destruction of bronchiolar and septal elastic fibers
    Explanation
    The patient suffers from emphysema, in which neutrophils enter the lung parenchyma and secrete elevated levels of elastase, leading to the destruction of the bronchiolar and alveolar septal elastic tissue support. The destruction of the elasticity in emphysema leads to diminished breath sounds. This is coupled with faint high-pitched rhonchi at the end of expiration and a hyperresonant percussion note. The rhonchi are adventitious (not normally present) sounds that may be high pitched, generally because of bronchospasm, or low pitched, generally because of the presence of airway secretions. Emphysema is a disease characterized by parenchymal tissue destruction and, therefore, is not associated with adventitious breath sounds. However, because most emphysema is due to cigarette smoking, there is almost always some degree of chronic bronchitis, and therefore, rhonchi can be auscultated.

    There are genetic and environmental causes of emphysema. The environmental causes include smoking and air pollution, whereas deficiency in 1-antitrypsin (antiprotease) activity is the genetic cause of the disease. The balance between normal elastase-elastin production and protease-antiprotease activity is altered in emphysema. Persons with a deficiency in 1-antitrypsin activity lack sufficient antiprotease activity to counteract neutrophil-derived elastase. When there is an increase in the entry and activation of neutrophils in the alveolar space, more elastase is released, and elastic structures are destroyed. In smoking there is an increase in the number of neutrophils and macrophages in alveoli and increased elastase activity from neutrophils and macrophages. These changes are coupled with a decrease in antielastase activity because of oxidants in cigarette smoke and antioxidants released from the increased numbers of neutrophils. The increased protease activity causes breakdown of the alveolar walls and dissolution of elastin in the bronchiolar walls. The loss of tethering of the bronchioles to the lung parenchyma leads to their collapse. The bronchioles, unlike the trachea and bronchi, do not contain hyaline cartilage. A relatively thick layer of smooth muscle is found in the bronchioles, but the bronchioles are tethered to the lung parenchyma by elastic tissue, which plays a key role in the stretch and recoil of the lungs during inhalation and exhalation.

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  • 44. 

    A teenage girl presents in the emergency room with paroxysms of dyspnea, cough, and wheezing. Her parents indicate that she has had these "attacks" during the past winter, and that they have worsened and become more frequent during the spring allergy season. Which of the following cell types is correctly matched to a function it may perform in this patient's disease?

    • A.

      Alveolar macrophages, enhanced mucociliary transport

    • B.

      Plasma cells, bronchoconstriction

    • C.

      Eosinophils, bronchodilation

    • D.

      Goblet cells, hyposecretion

    • E.

      Mast cells, edema

    Correct Answer
    E. Mast cells, edema
    Explanation
    The teenage patient is suffering from an asthmatic attack, probably allergen-induced. Mast cells are a key player in this airway disease. Mast cells in the bronchioles are stimulated to release histamine and heparin that induce the contraction of smooth bronchiolar muscle and edema in the wall. If the bronchoconstriction is chronic, the long-term result is thickening of the bronchiolar musculature. Other cells involved in asthma include eosinophils, neutrophils, macrophages, and lymphocytes, which signal to each other through a complex cytokine network. Mediators released include bradykinin, leukotrienes, and prostaglandins, which enhance bronchoconstriction, vascular congestion, and edema. The airway epithelium also is involved in response to and release of mediators. These muscle changes are usually accompanied by goblet cell hypersecretion of a viscous mucus, which can obstruct the airway. Eosinophils release proteins that destroy the airway epithelium (releasing Creola bodies). T lymphocytes are also present in more severe "attacks" and, along with B lymphocytes, may play a role in the initiation of allergic asthma. T lymphocytes also release cytokines that activate cell-mediated immunity pathways. Mucociliary transport is active in the trachea and bronchi; alveolar macrophages do not play a role in that process.

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  • 45. 

    Signal transduction in the epithelium lining the region with the arrow differs from that in rod cells stimulated by light in which of the following ways?

    • A.

      Sodium influx into receptor cells

    • B.

      Involvement of specific G proteins

    • C.

      Stimulation of a cyclic nucleotide

    • D.

      Stimulation leading to depolarization

    • E.

      Bypass of the protein kinase system

    Correct Answer
    D. Stimulation leading to depolarization
    Explanation
    The region shown on the MRI is the olfactory area lined by the olfactory epithelium. The response of rod cells to light causes hyperpolarization, whereas olfactory stimuli result in depolarization. The olfactory epithelium and rod cells are two examples of signal transduction that bypass a protein kinase system. In the case of the olfactory epithelium, an odorant molecule binds to an odor-specific transmembrane receptor found on the modified cilia at the apical surface. The binding activates an odorant-specific G protein (Golf), which binds GTP. The resulting dissociation of the subunit stimulates adenylate cyclase to produce cyclic AMP. Cyclic AMP directly stimulates the opening of the cation channels on the membrane of the bipolar olfactory receptor cells, leading to Na+ influx. The resulting change in membrane potential (depolarization) is transmitted from the modified cilia to the olfactory vesicle through the neuron to the basal axon. Axonal processes traverse the lamina propria as the olfactory nerve and pass through the cribriform plate of the ethmoid to terminate in the olfactory bulb. In the case of the rod, the cyclic nucleotide involved is cGMP

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  • 46. 

    The structure indicated by the arrow in the photomicrograph secretes its product by which of the following mechanisms?

    • A.

      Holocrine

    • B.

      Merocrine

    • C.

      Apocrine

    • D.

      Endocrine

    • E.

      Autocrine

    Correct Answer
    A. Holocrine
    Explanation
    The structure marked with the arrow is a sebaceous gland, which is located in the dermis and associated with the hair follicles. The sebaceous glands are holocrine (i.e., they shed the cell along with the secretory product). Sebum is a lipid product released into a duct that terminates in a hair follicle. The photomicrograph represents a microscopic section obtained from thin skin. The presence of sebaceous glands identifies the section as thin skin. Sebaceous glands and hair follicles are not found in thick skin. Another difference between thick and thin skin is the virtual absence of the stratum lucidum in thin skin.

    The sweat glands are of two different types: merocrine glands and apocrine glands. The merocrine glands release their secretion through exocytosis with conservation of membrane. In the anal, areolar, and axillary regions, the sweat glands are of the apocrine type and empty into the hair follicles. In apocrine glands, the apical part of the cell is released with the secretion. Endocrine secretion occurs into the blood; autocrine secretion is self-stimulation. For example, activated T cells stimulate their own proliferation by secreting IL-2 and synthesizing IL-2 receptors that bind the IL-2.

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  • 47. 

    In the photomicrograph below from the small intestinal epithelium, the area indicated between the arrows is the site of which of the following?

    • A.

      Glucose and galactose cotransporters

    • B.

      Passive diffusion of monosaccharides

    • C.

      Uptake of triglycerides by endocytosis

    • D.

      Release of chylomicra by exocytosis

    • E.

      Active transport of gycerol into the enterocyte

    Correct Answer
    A. Glucose and galactose cotransporters
    Explanation
    The structure between the arrows is the brush border containing microvilli. It is the site of monosaccharide cotransport of glucose and galactose, brush border enzymes (e.g., monosaccharidases), and enterokinase, that is important for cleavage of pancreatic zymogens (e.g., trypsinogen) to their active form.

    Digestion of lipids occurs through the action of bile (from the liver and bile duct) and lipase (from the pancreas). Bile serves to emulsify the lipid to form micelles, whereas lipase breaks down the lipid from triglycerides to fatty acids, glycerol, and monoglycerides. These three breakdown products diffuse freely across the microvilli to enter the apical portion of the enterocyte by passive diffusion. Triglycerides are resynthesized in the smooth endoplasmic reticulum. Proteins are synthesized in the RER and are combined with sugar and lipid portions in the Golgi to form glycoproteins and lipoproteins. These two types of molecules form the coverings of the triglyceride cores of the chylomicra. The chylomicra are released at the basolateral membranes by exocytosis into the lacteals. From the lacteals, the chylomicra travel into the cisterna chyli and eventually into the venous system by way of the thoracic duct. Digestion of fat occurs to a greater extent in the duodenum and jejunum than in the ileum. Sugars are broken down by amylase in the oral cavity, with continued digestion by monosaccharidases on the brush border. Proteins are broken down by pepsinogen in the stomach with continued breakdown in the small intestine by the enzymes of the pancreatic jucie (e.g., trypsin, chymotrypsin, and carboxypeptidases). The products of protein digestion are amino acids that are actively transported by transporters also located in the brush border.

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  • 48. 

    As saliva passes through the duct system, which of the following changes occurs?

    • A.

      Active secretion of Na+

    • B.

      Secretion of Cl2–

    • C.

      Absorption of HCO3–

    • D.

      Secretion of K+

    • E.

      Absorption of Ca2+

    Correct Answer
    D. Secretion of K+
    Explanation
    The change from isotonic to hypotonic saliva involves the secretion of K+ from the striated duct cells. The primary secretion produced by the acinar cells consists of amylase, mucus, and ions in the same concentrations as those of the extracellular fluid. In the duct system, Na+ is actively absorbed from the lumen of the ducts, Cl– is passively absorbed, K+ is actively secreted, and HCO3– is secreted. The result is a hypotonic sodium and chloride concentration and a hypertonic potassium concentration.

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  • 49. 

    In this photomicrograph, the cell labeled with the arrow functions in which of the following ways?

    • A.

      Transport chylomicra from the small intestine

    • B.

      Phagocytose antigen-antibody complexes

    • C.

      Synthesize IgA

    • D.

      Present antigen

    • E.

      Inhibit thrombogenic activity

    Correct Answer
    C. Synthesize IgA
    Explanation
    The cell labeled with the arrow is a plasma cell (in the lamina propria) that synthesizes immunoglobulins (antibodies), including secretory immunoglobulin (IgA). IgA is coupled with secretory component synthesized by the enterocyte to protect the IgA from luminal proteolytic contents. The photomicrograph shows a section through a portion of a villus in the small intestine. The core of the villus is occupied by blood and lymphatic capillaries (lacteals). The cell designated by the arrow is within a lymphatic lacteal and is classified as a plasma cell by its characteristic round, eccentric nucleus with heterochromatin arrayed in a clock-face pattern. A distinct, pale-staining perinuclear clear area (region of the Golgi apparatus) is also present. Other plasma cells and smaller lymphocytes are present within the lacteal. The organ is identified as small intestine by the simple columnar epithelium with a brush border (microvilli) and the presence of goblet cells. Macrophages and M cells present antigen in the small intestine. The M cells are found in the follicle-associated epithelium of the Peyer's patches and sample antigens from the lumen. Endocytosis of luminal antigens by M cells is followed by presentation of antigen to intraepithelial lymphocytes. The Peyer's patches form part of the gut-associated lymphoid tissue (GALT). Lacteals transport chylomicra, eosinophils phagocytose antigen-antibody complexes, and vascular endothelial cells possess antithrombogenic properties.

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  • 50. 

    A pathologist views the following tissues (A and B) in a biopsy. She determines that the tissues are normal. The presence of both of these tissues indicates that the sample was taken from the region of the junction between which of the following?

    • A.

      Anal canal and rectum

    • B.

      Esophagus and stomach

    • C.

      Skin of the face and mucous epithelium of the lip

    • D.

      Stomach and duodenum

    • E.

      Vagina and cervix

    Correct Answer
    A. Anal canal and rectum
    Explanation
    Photomicrographs A and B show two distinctly different types of epithelium: stratified squamous epithelium of the anus (top panel) and crypts (without villi) of the rectum (lower panel). The anus has anal valves and an absence of the muscularis mucosa. The esophageal-cardiac junction also represents a junction between stratified squamous and simple columnar epithelium, but the cardiac portion of the stomach forms the mucus-secreting cardiac glands with no goblet cells. The junction of the stomach (pylorus) and duodenum represents the juncture of two simple columnar epithelia, the pylorus containing the short (compared with fundus) pyloric glands and the duodenum with crypts and villi as well as the submucosal Brunner's glands. Skin is keratinized. The cervical mucosa contains extensive cervical glands, and the vaginal epithelium is keratinized. In vagina and cervix, the GI tract pattern [epithelium, connective tissue (CT), muscle, CT, muscle, CT] is not present.

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  • Mar 21, 2023
    Quiz Edited by
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