Micro Final Quiz Block I
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A 70-year-old woman has sustained third-degree burns over a significant area of her body. Despite appropriate burn care in the hospital, she spiked a fever to 39°C, and the nurse reports blue-green pus on the dressing covering the burned area. Gram stain of the pus reveals gram-negative rods, and antibiotic sensitivity tests show resistance to most antibiotics. Which one of the following organisms is MOST likely to cause this disease?
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Nocardia asteroides
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Vibrio vulnificus
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Bacteroides fragilis
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Haemophilus influenzae
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Pseudomonas aeruginosa
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This quiz evaluates knowledge on infectious agents and related diseases, focusing on microbiological diagnostics and immunologic factors, vital for medical students and professionals.
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- 2.
A 20-year-old woman has had several episodes of high fever, shaking chills, and a severe headache. She has a hematocrit of 30%. She has recently returned from Africa, where she was a Peace Corps volunteer. Which one of the following is MOST likely to be seen in the blood smear sample from this patient?
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Acid-fast rods
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Banana-shaped gametocytes
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Nonseptate hyphae
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Spherules
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Tachyzoites
Correct Answer
A. Banana-shaped gametocytesExplanation
Banana-shaped gametocytes are characteristic of Plasmodium falciparum, the causative agent of malaria. Malaria is a mosquito-borne infectious disease commonly found in Africa. The symptoms described in the question, such as high fever, shaking chills, and severe headache, are consistent with malaria. The low hematocrit level may be indicative of anemia, which is commonly associated with malaria infection. Therefore, the presence of banana-shaped gametocytes in the blood smear sample is most likely in this patient.Rate this question:
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- 3.
Certain microorganisms, such as the protozoan Trypanosoma and the bacterium Neisseria gonorrhoeae, can change their surface antigens quite frequently. This allows the organisms to evade our host defenses. Which one of the following BEST explains how this frequent change in antigenicity occurs?
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It is due to the transposition of existing genes into anactive expression site
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It is due to the acquisition of new fertility plasmids bytransduction
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It is due to conjugation, during which the recipientobtains new chromosomal genes
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It is due to new mutations that occur at “hot spots” inthe genome.
Correct Answer
A. It is due to the transposition of existing genes into anactive expression siteExplanation
The frequent change in antigenicity occurs due to the transposition of existing genes into an active expression site. Transposition is a process in which genetic elements, such as transposons, move from one location in the genome to another. In this case, the genes responsible for surface antigens are transposed into an active expression site, leading to a change in the antigens displayed on the microorganism's surface. This allows the microorganism to evade host defenses as the immune system may not recognize the new antigens.Rate this question:
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- 4.
A 60-year-old woman had an adenocarcinoma of the colon that was surgically removed. Several blood transfusions were given, and she did well until 3 weeks after surgery, when fever, vomiting, and diarrhea began. Blood and stool cultures were negative for bacteria, and the tests for Clostridium difficile and hepatitis B surface antigen were negative. A liver biopsy revealed intranuclear inclusion bodies. Which one of the following is the MOST likely cause?
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Cytomegalovirus
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Dengue virus
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Hepatitis A virus
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Rotavirus
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Yellow fever virus
Correct Answer
A. CytomegalovirusExplanation
The most likely cause of the symptoms described in the patient is cytomegalovirus (CMV) infection. CMV is a common virus that can cause severe illness in immunocompromised individuals, such as this post-surgical patient. The symptoms of fever, vomiting, and diarrhea, along with negative bacterial cultures and the presence of intranuclear inclusion bodies in the liver biopsy, are consistent with CMV infection. The other viral options listed (Dengue virus, Hepatitis A virus, Rotavirus, and Yellow fever virus) are less likely to cause these specific symptoms in this patient population.Rate this question:
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- 5.
Mycobacterium tuberculosis and Mycobacterium avium-complex (MAC) are important causes of disease, especially in immunocompromised patients. (MAC is also known as Mycobacterium avium-intracellulare.) Regarding MTB and MAC, which one of the following statements is the MOST accurate?
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Cell-mediated immunity is the most important host defense mechanism against MTB, whereas antibodymediated immunity is the most important host defense mechanism against MAC.
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In the clinical laboratory, MAC can be distinguished from MTB by the fact that MAC forms colonies in 7 days, whereas MTB does not.
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Multidrug-resistant strains of MAC are much less common than multidrug-resistant strains of MTB.
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MAC is found in the environment and is not transmitted from person to person, whereas MTB is found in humans and is transmitted from person to person.
Correct Answer
A. MAC is found in the environment and is not transmitted from person to person, whereas MTB is found in humans and is transmitted from person to person.Explanation
The correct answer states that MAC is found in the environment and is not transmitted from person to person, whereas MTB is found in humans and is transmitted from person to person. This statement accurately distinguishes the transmission patterns of the two bacteria. MTB is primarily transmitted through respiratory droplets when an infected person coughs or sneezes, while MAC is acquired through environmental exposure, such as contaminated water or soil. Understanding the modes of transmission is crucial for implementing appropriate infection control measures and preventing the spread of these diseases.Rate this question:
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- 6.
Endotoxin is an important underlying cause of septic shock and death, especially in hospitalized patients. Regarding endotoxin, which one of the following is the MOST accurate?
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It acts by phosphorylating the G stimulating protein
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It is a polypeptide with an A-B subunit configuration
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It induces the synthesis of tumor necrosis factor
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It is found primarily in gram-positive rods
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It can be treated with formaldehyde to form an effectivetoxoid vaccine
Correct Answer
A. It induces the synthesis of tumor necrosis factorExplanation
Endotoxin induces the synthesis of tumor necrosis factor, which is a pro-inflammatory cytokine. This cytokine plays a crucial role in the immune response to infection and inflammation. Endotoxin is a lipid component of the outer membrane of gram-negative bacteria and is released upon bacterial cell death or lysis. Its interaction with immune cells triggers the release of various inflammatory mediators, including tumor necrosis factor, leading to the development of septic shock and potentially death.Rate this question:
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- 7.
Which one of the following answer choices consists of bacteria, BOTH of which produce exotoxins that act by ADP-ribosylation?
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Salmonella typhi and Vibrio cholerae
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Vibrio cholerae and Corynebacterium diphtheriae
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Salmonella typhi and Clostridium perfringens
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Corynebacterium diphtheriae and Staphylococcusaureus
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Clostridium perfringens and Streptococcus pyogenes
Correct Answer
A. Vibrio cholerae and Corynebacterium diphtheriaeExplanation
Vibrio cholerae and Corynebacterium diphtheriae both produce exotoxins that act by ADP-ribosylation. ADP-ribosylation is a process in which an ADP-ribose molecule is added to a target protein, altering its function. In the case of Vibrio cholerae, the exotoxin known as cholera toxin ADP-ribosylates a protein involved in regulating fluid balance in the intestines, leading to severe diarrhea. Corynebacterium diphtheriae produces the exotoxin known as diphtheria toxin, which ADP-ribosylates a protein involved in protein synthesis, leading to the formation of a pseudomembrane in the throat and other symptoms of diphtheria.Rate this question:
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- 8.
Which one of the following is MOST likely to induce an IgM antibody response without the participation of helper T cells?
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Bacterial capsular polysaccharide
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Toxic shock syndrome toxin
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Penicillin–bovine serum albumin (BSA) complex
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Tetanus toxoid
Correct Answer
A. Bacterial capsular polysaccharideExplanation
Bacterial capsular polysaccharide is most likely to induce an IgM antibody response without the participation of helper T cells because polysaccharides are T-cell independent antigens. T-cell independent antigens can directly activate B cells without the need for T cell help. Bacterial capsular polysaccharides have repeating units of carbohydrate molecules that can directly stimulate B cells, leading to the production of IgM antibodies. This response does not require the recognition and activation of helper T cells.Rate this question:
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- 9.
Which one of the following diseases is MOST likely to be caused by a delayed hypersensitivity reaction?
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Serum sickness
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Poststreptococcal glomerulonephritis
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Systemic lupus erythematosus
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Hemolytic disease of the newborn
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Contact dermatitis
Correct Answer
A. Contact dermatitisExplanation
Contact dermatitis is most likely to be caused by a delayed hypersensitivity reaction. This condition occurs when the skin comes into contact with an allergen or irritant, leading to an inflammatory response. The immune system recognizes the allergen or irritant as foreign and triggers a delayed hypersensitivity reaction, resulting in redness, itching, and inflammation of the skin. This reaction typically occurs within 24-48 hours after exposure to the allergen or irritant.Rate this question:
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- 10.
A 60-year-old woman had surgery for ovarian carcinoma 4 days ago and has an indwelling urinary catheter in place. She now spikes a fever to 39°C and has cloudy urine in the collection bottle. Gram stain of the urine shows many polys and gram-positive cocci in chains. Which one of the following would be the MOST likely finding in the urine culture?
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α-Hemolytic colonies on the blood agar plate that are optochin-sensitive
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β-Hemolytic colonies on the blood agar plate that are bacitracin-sensitive
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β-Hemolytic colonies on the blood agar plate that hydrolyze hippurate
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Nonhemolytic colonies on the blood agar plate that grow in 6.5% sodium chloride
Correct Answer
A. Nonhemolytic colonies on the blood agar plate that grow in 6.5% sodium chlorideExplanation
The presence of cloudy urine and a fever in a patient with an indwelling urinary catheter suggests a urinary tract infection. Gram stain showing gram-positive cocci in chains indicates a possible infection with Streptococcus species. The most likely finding in the urine culture would be nonhemolytic colonies on the blood agar plate that grow in 6.5% sodium chloride, which is characteristic of Enterococcus species. Enterococcus is a common cause of urinary tract infections and is known to grow in high salt concentrations.Rate this question:
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- 11.
Regarding the diagnosis, treatment, and prevention of HIV, which one of the following is the MOST accurate?
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The drug zidovudine (AZT) is a “chain terminating” drug; that is, it inhibits the growing polypeptide chain by causing misreading of the viral mRNA.
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The drug lamivudine (3TC) acts by binding to the integrase, which prevents integration of the viral DNA into cellular DNA.
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In the screening test for HIV infection, the enzyme linked immunosorbent assay (ELISA) test detects the presence of antibody to the p24 protein of HIV.
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A major limitation to our ability to produce a vaccine against HIV is that there are many serologic types of the viral p24 protein.
Correct Answer
A. In the screening test for HIV infection, the enzyme linked immunosorbent assay (ELISA) test detects the presence of antibody to the p24 protein of HIV.Explanation
The correct answer states that the enzyme linked immunosorbent assay (ELISA) test detects the presence of antibody to the p24 protein of HIV in the screening test for HIV infection. This is the most accurate statement because ELISA is a commonly used diagnostic test for HIV and it is designed to detect antibodies produced by the immune system in response to HIV infection. The p24 protein is a viral protein that is present during the early stages of HIV infection, making it a useful target for detection in the ELISA test.Rate this question:
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- 12.
Your patient is a 20-year-old man with a urethral discharge. Gram stain of the pus reveals many neutrophils but no bacteria. Which one of the following organisms is the MOST likely cause?
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Treponema pallidum
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Haemophilus ducreyi
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Mycobacterium marinum
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Candida albicans
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Chlamydia trachomatis
Correct Answer
A. Chlamydia trachomatisExplanation
Chlamydia trachomatis is the most likely cause of the urethral discharge in this patient. The presence of many neutrophils but no bacteria on the Gram stain suggests an infection caused by an intracellular organism, and Chlamydia trachomatis is known to cause urethritis and urethral discharge. Treponema pallidum can cause a urethral discharge, but it is typically associated with other symptoms such as a painless genital ulcer. Haemophilus ducreyi can cause a painful genital ulcer with a purulent discharge, but it would likely be seen on the Gram stain. Mycobacterium marinum is not typically associated with urethral discharge. Candida albicans can cause urethritis, but it is less common compared to Chlamydia trachomatis.Rate this question:
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- 13.
An outbreak of postsurgical wound infections caused by Staphylococcus aureus has occurred. The infection control team was asked to determine whether the organism could be carried by one of the operating room personnel. Using your knowledge of normal flora, which one of the following body sites is the MOST likely location for this organism?
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Colon
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Gingival crevice
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Nose
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Throat
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Vagina
Correct Answer
A. NoseExplanation
Staphylococcus aureus is commonly found in the nasal passages of healthy individuals, making the nose the most likely location for this organism. The nasal passages serve as a reservoir for S. aureus, and it can easily be transmitted from the nose to other body sites or to patients during surgery. Therefore, it is important to investigate the nasal carriage of operating room personnel in order to identify the source of the infection and prevent further spread.Rate this question:
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- 14.
A 75-year-old man with substernal chest pain was found to have angina pectoris caused by syphilitic aortitis that affected his coronary arteries. Of the following, which one is the MOST likely way that the diagnosis of syphilis was made?
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Blood culture
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Culture on Thayer-Martin medium (chocolate agar with antibiotics)
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Detecting antibodies to cardiolipin in his blood
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Detecting treponemal antigen in his blood
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Western blot assay
Correct Answer
A. Detecting antibodies to cardiolipin in his bloodExplanation
The most likely way that the diagnosis of syphilis was made in this case is by detecting antibodies to cardiolipin in the patient's blood. Cardiolipin is a phospholipid that is found in the cell membranes of Treponema pallidum, the bacteria that causes syphilis. The presence of antibodies to cardiolipin indicates an immune response to the bacteria, suggesting an active or past infection. This is a common method used to diagnose syphilis in clinical practice. Blood culture, culture on Thayer-Martin medium, detecting treponemal antigen in the blood, and Western blot assay are not typically used for the diagnosis of syphilis.Rate this question:
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- 15.
A 4-year-old girl has papular and pustular lesions on her face. The lesions are exuding a honey-colored serous fluid. You make a clinical diagnosis of impetigo. A Gram stain of the exudate reveals gram-positive cocci in chains, and a culture reveals β-hemolytic colonies on blood agar. For which one of the following sequelae is she MOST at risk?
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Bloody diarrhea
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Blurred vision
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Paralysis of the facial nerve (Bell’s palsy)
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Red blood cells and albumin in her urine
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Rusty-colored sputum
Correct Answer
A. Red blood cells and albumin in her urineExplanation
The correct answer is "Red blood cells and albumin in her urine" because impetigo can lead to post-streptococcal glomerulonephritis, which is characterized by inflammation of the glomeruli in the kidneys. This can result in the leakage of red blood cells and albumin into the urine.Rate this question:
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- 16.
A 35-year-old man with a CD4 count of 50 presents with a skin nodule on his chest. The nodule is about 3 cm in diameter and is not red, hot, or tender. He says it has been slowly growing bigger for the past 3 weeks. You biopsy the nodule, and the pathologist calls to say that the patient has disseminated cryptococcosis. Which one of the following is the BEST description of what the pathologist saw in the biopsy specimen?
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Spherules
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Non-septate hyphae
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Germ tubes
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Budding yeasts with a thick capsule
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Septate hyphae with low-angle branching
Correct Answer
A. Budding yeasts with a thick capsuleExplanation
The pathologist saw budding yeasts with a thick capsule in the biopsy specimen. This finding is consistent with disseminated cryptococcosis, which is caused by the fungus Cryptococcus neoformans. The thick capsule is a characteristic feature of this fungus and helps it evade the host immune response. Disseminated cryptococcosis is commonly seen in immunocompromised individuals, such as those with a low CD4 count, as in this case. The other options listed are not associated with cryptococcosis.Rate this question:
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- 17.
A family of four from New Jersey has embarked on a vacation in the Rocky Mountains. All four required a 24 to 48 h acclimation to the high altitude, as all were breathing at a rapid pace until the acclimation took effect. In addition to increasing the number of red blood cells in circulation, what other compensatory mechanism occurred within the red blood cell during this acclimation period?
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Increased synthesis of lactic acid
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Decreased synthesis of lactic acid
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Increased synthesis of 2,3-bisphosphoglycerate
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Decreased synthesis of 2,3-bisphosphoglycerate
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Decreased degradation of bilirubin, producing less carbon monoxide
Correct Answer
A. Increased synthesis of 2,3-bisphosphoglycerateExplanation
During the acclimation period to high altitude, the family from New Jersey experienced an increase in the synthesis of 2,3-bisphosphoglycerate (2,3-BPG) within their red blood cells. This is a compensatory mechanism that helps to enhance oxygen delivery to tissues. By increasing the synthesis of 2,3-BPG, the red blood cells are able to release oxygen more readily to the tissues in the body. This is important at high altitudes where the air is thinner and oxygen levels are lower. Thus, the increased synthesis of 2,3-BPG helps the family adapt to the high altitude conditions and maintain adequate oxygen supply to their tissues.Rate this question:
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- 18.
In the 1800s, British sailors on long sea journeys developed sore and bleeding gums, sometimes to the point that their teeth would loosen and fall out. The introduction of limes to their diets helped to prevent these occurrences. The biochemical step that was lacking in these sailors was which of the following?
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Creating lysine cross-links in collagen
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Mobilization of calcium into bone
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Hydroxylation of proline residues in collagen
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Glycosylation of fibrillin
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Conversion of glycine to proline in collagen
Correct Answer
A. Hydroxylation of proline residues in collagenExplanation
The sailors in the 1800s developed sore and bleeding gums, which indicates a deficiency in collagen, the protein responsible for maintaining the health of gums and teeth. Hydroxylation of proline residues in collagen is a crucial step in its formation, and its deficiency can lead to weakened collagen, causing the symptoms observed in the sailors. The introduction of limes to their diets, rich in vitamin C, helped prevent these symptoms as vitamin C is necessary for the hydroxylation of proline residues in collagen.Rate this question:
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- 19.
A 9-year-old girl was playing soccer when she began to limp. She has a pain in her leg and points to her upper thigh when asked where it hurts. Her temperature is 101°F. X-ray of the femur reveals that the periosteum is eroded. You order a blood culture. Which one of the following would be the MOST likely blood culture findings?
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Gram-negative rods that grow on EMB agar, formingpurple colonies and a green sheen
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Gram-positive cocci that grow on blood agar, causing aclear zone of hemolysis, and are coagulase-positive
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Gram-positive rods that grow only anaerobically andform a double zone of hemolysis on blood agar
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Gram-negative diplococci that grow on chocolate agar,are oxidase-positive, and ferment maltose
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Gram-positive cocci that grow on blood agar, causinga green zone of hemolysis, and are not inhibited byoptochin and bile
Correct Answer
A. Gram-positive cocci that grow on blood agar, causing aclear zone of hemolysis, and are coagulase-positiveExplanation
The most likely blood culture findings would be Gram-positive cocci that grow on blood agar, causing a clear zone of hemolysis, and are coagulase-positive. This is because the description of the bacteria matches the characteristics of Staphylococcus aureus, which is a common cause of osteomyelitis in children. The erosion of the periosteum suggests an infection in the bone, and the presence of a fever further supports the diagnosis of an infectious process.Rate this question:
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- 20.
Your summer research project is to study the viruses that cause upper respiratory tract infections. You have isolated a virus from a patient’s throat and find that its genome is RNA. Furthermore, you find that the genome is the complement of viral mRNA within the infected cell. Of the following, which one is the MOST appropriate conclusion you could draw?
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The virion contains a polymerase
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The purified genome RNA is infectious
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The genome RNA is segmented
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A single-stranded DNA is synthesized during replication
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The genome RNA encodes a precursor polypeptide thatmust be cleaved by a protease.
Correct Answer
A. The virion contains a polymeraseExplanation
The presence of a viral genome that is the complement of viral mRNA suggests that the virus contains a polymerase enzyme. This enzyme is responsible for synthesizing RNA from an RNA template, indicating that the virion contains a polymerase.Rate this question:
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- 21.
A 25-year-old man has a history of four episodes of boils in the last year. Boils are abscesses caused by Staphylococcus aureus. Which one of the following is MOST likely to be the underlying immunologic factor that predisposes him to multiple episodes of boils?
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A deficient amount of the C8 component of complementin his plasma
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An inability of his macrophages to present antigen inassociation with class I MHC proteins
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A failure to release granzymes from his cytotoxic T cells
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An insufficient amount of IgG in his plasma
Correct Answer
A. An insufficient amount of IgG in his plasmaExplanation
The most likely underlying immunologic factor that predisposes the man to multiple episodes of boils is an insufficient amount of IgG in his plasma. IgG is an important antibody that plays a crucial role in immune defense against bacterial infections, including Staphylococcus aureus. A deficiency in IgG can weaken the immune response and make the individual more susceptible to recurrent infections.Rate this question:
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- 22.
Your patient has been treated for endocarditis with penicillin G for the past 2 weeks. She now has a fever and maculopapular erythematous rash over her chest and abdomen. A urinalysis shows significant protein in the urine. If the fever, rash, and proteinuria are immunologic in origin, which one of the following is MOST likely to be involved?
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IgG and complement
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IgE and histamine
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IL-2 and cytotoxic T cells
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Gamma interferon and macrophages
Correct Answer
A. IgG and complementExplanation
The patient's symptoms of fever, rash, and proteinuria are suggestive of an immune response. IgG and complement are most likely to be involved in this immunologic reaction. IgG is an antibody that plays a crucial role in the immune response, and complement is a group of proteins that work together with antibodies to enhance the immune response. The presence of significant protein in the urine suggests kidney involvement, which can occur in immune-mediated diseases such as immune complex-mediated glomerulonephritis. IgG and complement can form immune complexes that deposit in the kidney, leading to inflammation and proteinuria.Rate this question:
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- 23.
Regarding the function of chemokines in host defenses, which one of the following is the MOST accurate?
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Chemokines bind to the T-cell receptor outside of theantigen-binding site and activate many T cells.
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Chemokines induce gene switching in B cells thatincreases the amount of IgE synthesized, thereby predisposingto allergies
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Chemokines penetrate the membranes of target cellsduring attack by cytotoxic T cells
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Chemokines attract neutrophils to the site of bacterialinfection, thereby playing a role in the inflammatoryresponse
Correct Answer
A. Chemokines attract neutrophils to the site of bacterialinfection, thereby playing a role in the inflammatoryresponseExplanation
Chemokines are small proteins that play a crucial role in the immune system by attracting immune cells to the site of infection or inflammation. The correct answer states that chemokines attract neutrophils to the site of bacterial infection, which is accurate. Neutrophils are a type of white blood cell that are important for fighting bacterial infections. When chemokines are released at the site of infection, they act as signals for neutrophils to migrate towards the infected area, where they can eliminate the bacteria and contribute to the inflammatory response.Rate this question:
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- 24.
Regarding hepatitis C virus (HCV) and hepatitis D virus (HDV), which one of the following is MOST accurate?
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HCV is transmitted by blood, but HDV is not
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More than half of HCV infections result in a chroniccarrier state.
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There is an effective vaccine against HCV but notagainst HDV
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Both HCV and HDV are defective RNA viruses andrequire concurrent HBV infection to replicate
Correct Answer
A. More than half of HCV infections result in a chroniccarrier state.Explanation
More than half of HCV infections result in a chronic carrier state. This means that a significant portion of individuals infected with HCV will continue to carry the virus in their body for a prolonged period, even if they do not show any symptoms. This chronic carrier state can lead to long-term liver damage and increase the risk of developing liver cirrhosis or liver cancer. It is important to identify and treat individuals with chronic HCV infection to prevent further complications and to reduce the transmission of the virus to others.Rate this question:
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- 25.
Regarding the mode of action of antiviral drugs, which one of the following is MOST accurate?
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Amantadine inhibits influenza A virus by inhibiting theRNA polymerase carried by the virion
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Foscarnet inhibits varicella-zoster virus by inhibitingthe RNA polymerase carried by the virion
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Acyclovir action is greater in herpesvirus-infected cellsthan in uninfected cells because herpesvirus-infectedcells contain an enzyme that phosphorylates acyclovirvery efficiently
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Azidothymidine inhibits human immunodeficiencyvirus (HIV) by inhibiting viral mRNA synthesis moreefficiently than cellular mRNA synthesis
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Indinavir blocks HIV replication by inhibiting the proteaserequired for the envelope protein gp120 to bind tothe CD8 protein on the surface of the T cell
Correct Answer
A. Acyclovir action is greater in herpesvirus-infected cellsthan in uninfected cells because herpesvirus-infectedcells contain an enzyme that phosphorylates acyclovirvery efficientlyExplanation
Acyclovir is an antiviral drug that is specifically effective against herpesviruses. It is phosphorylated by a viral enzyme called thymidine kinase, which is present in herpesvirus-infected cells but not in uninfected cells. This phosphorylation process converts acyclovir into its active form, which inhibits viral DNA synthesis. Therefore, acyclovir's action is greater in herpesvirus-infected cells because the presence of the viral enzyme allows for more efficient conversion of the drug into its active form, leading to targeted inhibition of viral replication.Rate this question:
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- 26.
Members of the genus Mycobacterium stain better with the acid-fast stain than with the Gram stain. Which one of the following is the BEST explanation for this finding?
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They lack a cell wall; therefore, they cannot adsorb the crystal violet
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They have a very thin cell wall that does not retain the crystal violet
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They have a thick polysaccharide capsule that prevents entry of the iodine solution
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They have a large amount of lipid in their cell wall that prevents entry of the crystal violet
Correct Answer
A. They have a large amount of lipid in their cell wall that prevents entry of the crystal violetExplanation
The correct answer is that members of the genus Mycobacterium have a large amount of lipid in their cell wall that prevents entry of the crystal violet. This is because the acid-fast stain, which includes the use of a lipid-soluble dye, is able to penetrate the lipid-rich cell wall of Mycobacterium, while the Gram stain, which relies on the ability of the crystal violet dye to enter the cell wall, is less effective due to the lipid barrier. The other options provided do not accurately explain why Mycobacterium stains better with the acid-fast stain.Rate this question:
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- 27.
A 50-year-old man with a cadaveric renal transplant is rejecting the transplant despite immunosuppressive drugs. He is now in renal failure with a blood pH of 7.31. Yesterday, he developed a pain near his left eye that has become progressively more severe. On examination, his temperature is 37.5°C, and the skin near his eye is swollen and necrotic. Microscopic examination of a biopsy of the lesion reveals non-septate hyphae with right-angle branching. Which one of the following organisms is the MOST likely cause?
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Candida albicans
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Coccidioides immitis
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Cryptococcus neoformans
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Histoplasma capsulatum
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Mucor species
Correct Answer
A. Mucor speciesExplanation
The patient's presentation of a 50-year-old man with a cadaveric renal transplant experiencing transplant rejection despite immunosuppressive drugs suggests an opportunistic infection. The presence of non-septate hyphae with right-angle branching on microscopic examination of the biopsy is indicative of Mucor species, a filamentous fungus. Mucor species are known to cause invasive mucormycosis, which commonly affects immunocompromised individuals. This infection can lead to tissue necrosis, as seen in the patient's swollen and necrotic skin near his eye.Rate this question:
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- 28.
Your patient is a 40-year-old man with a history of confusion for the past 2 days and a grand mal seizure that occurred this morning. He is HIV antibody positive and has a CD4 count of 100/mL. On examination, his temperature is 37.5°C, and the findings of the remainder of the examination are within normal limits. Magnetic resonance imaging (MRI) reveals several “ring-enhancing” cavitary brain lesions. He has not traveled outside of the United States, is employed as the manager of a supermarket, is a strict vegetarian, and has several household pets, namely, a dog, a cat, a parrot, and a turtle. Which one of the following organisms is the MOST likely cause?
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Toxocara canis
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Toxoplasma gondii
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Taenia saginata
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Trichinella spiralis
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Trypanosoma cruzi
Correct Answer
A. Toxoplasma gondiiExplanation
The patient's history of confusion, seizure, and HIV infection with a low CD4 count suggests an opportunistic infection. The MRI findings of "ring-enhancing" cavitary brain lesions are characteristic of toxoplasmosis, which is caused by Toxoplasma gondii. Toxoplasmosis is commonly seen in individuals with compromised immune systems, such as those with HIV/AIDS. The patient's lifestyle and exposure to pets also increase the likelihood of Toxoplasma infection. Therefore, Toxoplasma gondii is the most likely cause in this case.Rate this question:
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- 29.
Regarding antimicrobial drugs that act by inhibiting nucleic acid synthesis in bacteria, which one of the following is the MOST accurate?
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Quinolones, such as ciprofloxacin, inhibit the RNA polymerase in bacteria by acting as nucleic acid analogues
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Rifampin inhibits the RNA polymerase in bacteria by binding to the enzyme and inhibiting messenger RNA synthesis
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Sulfonamides inhibit the DNA polymerase in bacteria by causing chain termination of the elongating strand
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Trimethoprim inhibits the DNA polymerase in bacteria by preventing the unwinding of double-stranded DNA
Correct Answer
A. Rifampin inhibits the RNA polymerase in bacteria by binding to the enzyme and inhibiting messenger RNA synthesisExplanation
Rifampin inhibits the RNA polymerase in bacteria by binding to the enzyme and inhibiting messenger RNA synthesis. This means that rifampin prevents the RNA polymerase enzyme from functioning properly, which in turn inhibits the synthesis of messenger RNA. Messenger RNA is responsible for carrying the genetic information from DNA to the ribosomes, where it is used to produce proteins. By inhibiting messenger RNA synthesis, rifampin effectively disrupts the bacteria's ability to produce essential proteins, leading to their inhibition or death.Rate this question:
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- 30.
A 22-year-old woman had fever to 100°F and anorexia for the past 2 days, and this morning she appears jaundiced. On examination, her liver is enlarged and tender. She has a total bilirubin of 5 mg/dL (normal, <1) and elevated transaminases. She received the complete course of the hepatitis B vaccine 2 years ago but has not had the hepatitis A vaccine. The results of her hepatitis serologies are as follows: HAVIgM negative, HAV-IgG positive, HBsAg negative, HBsAb positive, HBcAb negative, HCV-Ab positive. Of the following, which one is the MOST accurate?
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She probably has hepatitis A now, probably has not been infected with hepatitis B virus (HBV), and probably had hepatitis C in the past.
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She probably has hepatitis A now, probably has been infected with HBV in the past, and probably had hepatitis C in the past
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She has been infected with hepatitis A virus (HAV) in the past, probably has not been infected with HBV, and probably has hepatitis C now
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She has been infected with HAV in the past, probably has hepatitis B now, and probably had hepatitis C in the past
Correct Answer
A. She has been infected with hepatitis A virus (HAV) in the past, probably has not been infected with HBV, and probably has hepatitis C nowExplanation
The patient's negative HAVIgM and positive HAV-IgG indicate that she has been infected with hepatitis A virus (HAV) in the past. The negative HBsAg and positive HBsAb suggest that she has not been infected with hepatitis B virus (HBV). The positive HCV-Ab indicates that she probably has hepatitis C now. Therefore, the most accurate statement is that she has been infected with HAV in the past, probably has not been infected with HBV, and probably has hepatitis C now.Rate this question:
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- 31.
Two weeks after visiting a construction site in Ohio that was a former poultry farm, a 56-year-old Florida man with chronic myelogenous leukemia presents with fever of 39 °C and difficulty breathing. The illness began with flulike symptoms and progressed to nonproductive cough and dyspnea. A chest radiograph reveals diffuse pulmonary infiltrates and mediastinal lymphadenopathy. Results of a needle biopsy of lung tissue stained with methamine silver are shown in the accompanying photograph. What is the etiology of this man’s disease?
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Histoplasma capsulatum
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Nocardia asteroides
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Nocardia asteroides
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Streptococcus pneumoniae
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Toxoplasma gondii
Correct Answer
A. Histoplasma capsulatumExplanation
The correct answer is Histoplasma capsulatum. This is because the patient's symptoms, such as fever, difficulty breathing, and nonproductive cough, along with the findings on the chest radiograph, suggest a respiratory infection. The accompanying photograph shows lung tissue stained with methamine silver, which is used to visualize fungal organisms. Histoplasma capsulatum is a dimorphic fungus commonly found in soil contaminated with bird droppings, such as in poultry farms. It can cause respiratory symptoms and can be identified by its characteristic appearance on staining. Therefore, Histoplasma capsulatum is the most likely etiology of this man's disease.Rate this question:
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- 32.
A 15-year-old boy with acne is being treated with a topical antimicrobial. Which organism is associated with this disease and with which antimicrobial is he most likely being treated?
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Malassezia furfur/Ketoconazole
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Propionibacterium sp./Clindamycin
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Staphylococcus aureus/Cloxacillin
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Streptococcus pyogenes/Penicillin
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Trichosporon sp./Clotrimazole
Correct Answer
A. Propionibacterium sp./ClindamycinExplanation
The correct answer is Propionibacterium sp./Clindamycin. Acne is commonly caused by Propionibacterium acnes, a type of bacteria that is part of the normal skin flora. Clindamycin is an antimicrobial agent that is effective against Propionibacterium sp., making it a suitable treatment option for acne.Rate this question:
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- 33.
A 32-year-old prison inmate presented with fever of 38.5 °C, unexplained weight loss of 12 pounds, and a cough of 3 weeks duration. The cough was productive of bloody sputum. A sputum Gram stain failed to reveal any predominant organisms. An acid-fast stain of the sputum is shown in the figure. Chest radiographs showed cavitary lesions in the right upper lobe; however, an intradermal skin test showed a 5 mm area of induration after 72 h. In addition to the organism causing the respiratory symptoms, for which other organism should he be tested?
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Aspergillus fumigatus
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Histoplasma capsulatum
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Human immunodeficiency virus
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Klebsiella pneumoniae
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Streptococcus pneumoniae
Correct Answer
A. Human immunodeficiency virusExplanation
The patient's symptoms, including fever, weight loss, and productive cough with bloody sputum, along with the presence of cavitary lesions in the right upper lobe on chest radiographs, suggest a possible diagnosis of tuberculosis. The acid-fast stain of the sputum is used to detect acid-fast bacilli, which are characteristic of Mycobacterium tuberculosis, the causative agent of tuberculosis. However, the negative acid-fast stain does not rule out tuberculosis, as it can occur in some cases. Given the patient's risk factors as a prison inmate and the presence of respiratory symptoms, it is important to test for the human immunodeficiency virus (HIV) as well, as HIV infection increases the risk of developing tuberculosis.Rate this question:
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- 34.
A 56-year-old pathologist was taken to his family doctor by his son for he was showing mood changes, minor loss of memory, and decreased motor skills. During the patient history, it became clear that over the course of his career he had, on occasion, cut himself using the instruments he had been using on the cadavers he had been working on. A potential explanation for his symptoms is abnormal aggregation of which of the following proteins?
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Hemoglobin in the red blood cells
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Fibrillin in the extracellular compartments of the brain
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A truncated neuronal protein
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A misfolded form of a normal protein
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A truncated extracellular protein
Correct Answer
A. A misfolded form of a normal proteinExplanation
The potential explanation for the symptoms described in the case is a misfolded form of a normal protein. The history of the pathologist cutting himself with instruments used on cadavers suggests possible exposure to abnormal proteins. Misfolded proteins can accumulate in the brain and lead to neurodegenerative diseases, causing mood changes, memory loss, and motor skill impairment. This explanation aligns with the symptoms and the history provided in the case.Rate this question:
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- 35.
12-year-old girl had a seizure this morning and was rushed to the hospital. On examination, her temperature was 40°C, and she had no nuchal rigidity. Computed tomography (CT) scan revealed no abnormality. A spinal tap was done, and the protein and glucose were normal. Gram stain of the spinal fluid showed no organisms and no polys. She was treated with various antibiotics but became comatose and died 2 days later. The routine blood culture and spinal fluid culture grew no organism. On autopsy of the brain, eosinophilic inclusion bodies were seen in the cytoplasm of neurons. Of the following, which one is the MOST likely cause?
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Prions
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JC virus
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Rabies virus
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Parvovirus B19
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Herpes simplex virus type 1
Correct Answer
A. Rabies virusExplanation
The most likely cause of the 12-year-old girl's symptoms and subsequent death is the Rabies virus. This is indicated by the presence of eosinophilic inclusion bodies in the cytoplasm of neurons during autopsy. Rabies is a viral infection that affects the central nervous system and can cause seizures, high fever, and coma. The absence of organisms in the spinal fluid culture and the normal protein and glucose levels further support the viral etiology.Rate this question:
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- 36.
A 14-year-old girl with leukemia had been neutropenic for 5 weeks when she developed fever, cough, shortness of breath, pleuritic chest pain, and hemoptysis. The causative organism is seen in a lung biopsy stained with methenamine silver. With which antimicrobial should she be treated?
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Ciprofloxacin
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Imipenem–cilastatin
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Piperacillin–tazobactam
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Rifampin
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Voriconazole
Correct Answer
A. VoriconazoleExplanation
The 14-year-old girl with leukemia is presenting with symptoms of a lung infection, including fever, cough, shortness of breath, pleuritic chest pain, and hemoptysis. The causative organism is seen in a lung biopsy stained with methenamine silver. Voriconazole is the appropriate antimicrobial treatment in this case. Voriconazole is a broad-spectrum antifungal medication that is effective against a variety of fungal pathogens, including Aspergillus species, which is commonly associated with lung infections in immunocompromised individuals. Given the patient's neutropenic state and the presence of a fungal infection, voriconazole is the most suitable treatment option.Rate this question:
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- 37.
You are reading an article that says that otitis media is commonly caused by nonencapsulated strains of Haemophilus influenzae. You are surprised that nonencapsulated strains can cause this disease. Which one of the following BEST explains why your surprise is justified?
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Nonencapsulated strains would not have endotoxin
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Nonencapsulated strains cannot secrete exotoxin A
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Nonencapsulated strains should be easily phagocytized
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Nonencapsulated strains should be rapidly killed byultraviolet light
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Nonencapsulated strains should be susceptible to killingby cytotoxic T cells
Correct Answer
A. Nonencapsulated strains should be easily phagocytizedExplanation
Nonencapsulated strains should be easily phagocytized because the absence of a capsule makes it easier for phagocytes to recognize and engulf the bacteria. The capsule is a protective layer that helps bacteria evade the immune system, so without it, the nonencapsulated strains are more vulnerable to phagocytosis. This explains why the article's statement that nonencapsulated strains of Haemophilus influenzae can cause otitis media is surprising, as one would expect that these strains would be more easily eliminated by the immune system.Rate this question:
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- 38.
A 35-year-old man is HIV antibody positive and has a CD4 count of 50/mL (normal, 1000–1500). He has had a fever of 101°F for a few weeks and “feels tired all the time.” He has no other symptoms, and findings on physical examination are normal. Complete blood cell count, urinalysis, and chest X-ray are normal. Blood, stool, and urine cultures show no growth. A bone marrow biopsy reveals granulomas, and a culture grows an organism that is a budding yeast at 37°C but produces hyphae at 25°C. Of the following, which one is the MOST likely cause?
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Aspergillus fumigatus
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Cryptococcus neoformans
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Mucor species
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Histoplasma capsulatum
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Coccidioides immitis
Correct Answer
A. Histoplasma capsulatumExplanation
The patient's symptoms, including fever, fatigue, and granulomas on bone marrow biopsy, are consistent with disseminated histoplasmosis. Histoplasma capsulatum is a dimorphic fungus that grows as a budding yeast at 37°C and produces hyphae at 25°C, which matches the culture findings described in the question. Aspergillus fumigatus, Cryptococcus neoformans, Mucor species, and Coccidioides immitis are not known to exhibit this growth pattern.Rate this question:
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- 39.
A 70-year-old woman presents with rapid onset of fever to 39°C and a cough productive of greenish sputum. She is not hospitalized and not immunocompromised. A chest X-ray reveals a left lower lobe infiltrate. Of the following, which set of findings describes the MOST likely causative organism found in the sputum culture?
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Gram-positive diplococci that form an a-hemolyticcolony
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Gram-negative diplococci that form an oxidase-positivecolony
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Gram-positive rods that form a b-hemolytic colony
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Gram-negative rods that form an oxidase-positivecolony
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Gram-negative cocci that grow only anaerobically
Correct Answer
A. Gram-positive diplococci that form an a-hemolyticcolonyExplanation
The most likely causative organism found in the sputum culture is gram-positive diplococci that form an a-hemolytic colony. This is because the patient's symptoms, including fever, cough with greenish sputum, and a left lower lobe infiltrate on chest X-ray, are consistent with pneumonia. Gram-positive diplococci that form an a-hemolytic colony are characteristic of Streptococcus pneumoniae, which is a common cause of community-acquired pneumonia in non-immunocompromised individuals.Rate this question:
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- 40.
25-year-old pregnant woman in the third trimester comes to the emergency room saying that about 12 hours ago she began to feel feverish and weak. On examination, she has a temperature of 40°C but no other pertinent findings. A blood culture grows small gram-positive rods that cause β-hemolysis on a blood agar plate incubated in room air. Which one of the following bacteria is the MOST likely cause?
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Clostridium perfringens
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Streptococcus pyogenes
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Bacillus cereus
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Listeria monocytogenes
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Brucella abortus
Correct Answer
A. Listeria monocytogenesExplanation
Listeria monocytogenes is the most likely cause of the symptoms described in the pregnant woman. Listeria monocytogenes is a gram-positive rod-shaped bacterium that can cause infection in pregnant women, leading to flu-like symptoms such as fever and weakness. It is known to cause listeriosis, which can be particularly dangerous for pregnant women as it can lead to miscarriage, stillbirth, or severe illness in the newborn. The fact that the blood culture grew small gram-positive rods that cause beta-hemolysis on a blood agar plate further supports the likelihood of Listeria monocytogenes as the causative agent, as it is known to exhibit beta-hemolysis.Rate this question:
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- 41.
A 35-year-old man who is HIV antibody positive and has a CD4 count of 30 says, “I can’t remember the simplest things.” You are concerned about dementia. An MRI indicates several widely scattered lesions in the brain. Over the next 4 months, he develops visual field defects, becomes paralyzed, and dies. Autopsy reveals that many neurons of the brain have lost myelin and contain intranuclear inclusions. Electron microscopy reveals the inclusions contain nonenveloped viruses. Which one of the following viruses is the MOST likely cause?
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Adenovirus
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Cytomegalovirus
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Herpes simplex virus
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JC virus
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Coxsackie virus
Correct Answer
A. JC virusExplanation
The correct answer is JC virus. The patient's symptoms, along with the presence of widely scattered brain lesions, loss of myelin in neurons, and intranuclear inclusions containing nonenveloped viruses, are consistent with progressive multifocal leukoencephalopathy (PML), which is caused by JC virus. PML is a severe neurological disease that commonly affects individuals with compromised immune systems, such as those with HIV/AIDS. The JC virus infects oligodendrocytes, leading to demyelination and neurological deficits.Rate this question:
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- 42.
An 18-year-old woman preparing to enter college is required to be up-to-date on vaccinations. In addition to required childhood vaccinations, students entering college are often required to be vaccinated against which organism?
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Chlamydia trachomatis
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Hepatitis C virus
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Human papillomavirus
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Neisseria meningitidis
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Treponema pallidum
Correct Answer
A. Neisseria meningitidisExplanation
Students entering college are often required to be vaccinated against Neisseria meningitidis. This organism is responsible for causing meningococcal disease, which is a serious bacterial infection that can lead to meningitis and sepsis. College campuses can be high-risk environments for the spread of this bacteria, as students often live in close quarters and engage in activities that can facilitate transmission. Vaccination against Neisseria meningitidis helps to prevent outbreaks and protect the health of college students.Rate this question:
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- 43.
While removing trash and brush from a canal in Florida, a man is pierced with a thorn in one of his fingers. At the time of the injury, the wound seemed insignificant. After a few days, however, it becomes inflamed and swollen. Eventually, the wound becomes so painful that he is compelled to seek medical assistance. The physician astutely observes that with minimal pressure applied, greenish, sweets melling pus freely oozes from the lesion. Considering the clinical picture, which of the following etiologies is most likely?
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Actinomyces sp.
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Leptospira interrogans
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MRSA
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Pseudomonas aeruginosa
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Sporothrix schenckii
Correct Answer
A. Pseudomonas aeruginosaExplanation
The most likely etiology in this case is Pseudomonas aeruginosa. The presence of greenish, sweet-smelling pus is characteristic of Pseudomonas aeruginosa infection. This bacterium is commonly found in water and soil and can cause infections in wounds, especially when there is exposure to contaminated water. The delayed onset of symptoms and the progression to inflammation and swelling also support this diagnosis. Actinomyces sp., Leptospira interrogans, MRSA, and Sporothrix schenckii are less likely in this case based on the clinical presentation and history of exposure.Rate this question:
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- 44.
A teenager, new to your practice, comes in for a routine physical exam. His family had just moved to the city, and the boy had rarely seen a doctor before. Upon examination, you notice a high, arched palate, disproportionately long arms and fingers, a sunken chest, and mild scoliosis. The patient has been complaining of lack of breath while doing routine chores, and upon listening to his heart, you detect an aortic regurgitation murmur. Careful examination of the eyes is indicated by the figure below. Based on your physical exam and history, you are suspicious of an inborn error of metabolism in which of the following proteins?
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Collagen
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Fibrillin
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Elastin
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Dystrophin
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β-catenin
Correct Answer
A. FibrillinExplanation
The physical exam findings of a high, arched palate, disproportionately long arms and fingers, a sunken chest, and mild scoliosis, along with symptoms of lack of breath and an aortic regurgitation murmur, suggest a connective tissue disorder. Fibrillin is a protein that is essential for the formation of connective tissues, particularly in structures such as blood vessels, bones, and the heart. Mutations in the fibrillin gene can lead to a condition called Marfan syndrome, which is characterized by the physical features and symptoms described in the patient. Therefore, based on the physical exam and history, the suspicion of an inborn error of metabolism in fibrillin is justified.Rate this question:
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- 45.
While working an overnight shift in the emergency department you are called to see an 8-year-old boy who appears to have a fracture in his arm. Upon taking a history, you learn that this child has been to the ER multiple times for fractures, and the incidents that lead to the fracture would be described as mild trauma at best. X-rays indicate a number of healed fractures that the boy and his parents were unaware of (see example of arm X-ray below). Physical exam shows sky blue sclera. The parents then inform you that the child is taking bisphosphonates for his condition. The mechanism whereby the frequency of fractures is being reduced in this patient is which of the following?
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Increased synthesis of collagen
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Increased resorption of collagen
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Decreased synthesis of collagen
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Decreased resorption of collagen
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Increased synthesis of fibrillin
Correct Answer
A. Decreased resorption of collagenExplanation
The correct answer is "Decreased resorption of collagen." Bisphosphonates are medications commonly used to treat osteoporosis and other conditions characterized by increased bone resorption. These medications work by inhibiting the activity of osteoclasts, the cells responsible for breaking down bone tissue. By decreasing the resorption of collagen, which is a major component of bone, bisphosphonates help to reduce the frequency of fractures in patients with conditions like osteoporosis. The presence of healed fractures and sky blue sclera in this child suggests a possible underlying condition affecting collagen synthesis or resorption, and the use of bisphosphonates helps to decrease the resorption of collagen and prevent further fractures.Rate this question:
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Mar 15, 2023Quiz Edited by
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Jun 29, 2020Quiz Created by
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