1.
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?
Correct Answer
B. Gram-positive cocci that grow on blood agar, causing aclear zone of hemolysis, and are coagulase-positive
Explanation
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.
2.
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?
Correct Answer
A. The virion contains a polymerase
Explanation
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.
3.
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?
Correct Answer
D. An insufficient amount of IgG in his plasma
Explanation
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.
4.
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?
Correct Answer
C. Nonencapsulated strains should be easily pHagocytized
Explanation
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.
5.
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?
Correct Answer
D. Histoplasma capsulatum
Explanation
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.
6.
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?
Correct Answer
E. Pseudomonas aeruginosa
Explanation
The patient's symptoms, including the presence of blue-green pus, suggest an infection caused by Pseudomonas aeruginosa. This bacterium is known to cause infections in burn patients, particularly those with extensive burns. Pseudomonas aeruginosa is a gram-negative rod-shaped bacterium, which matches the description given in the gram stain. Additionally, the resistance to most antibiotics, as indicated by the antibiotic sensitivity tests, is consistent with Pseudomonas aeruginosa, as it is known to be intrinsically resistant to many antibiotics.
7.
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?
Correct Answer
B. Banana-shaped gametocytes
Explanation
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.
8.
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?
Correct Answer
A. It is due to the transposition of existing genes into anactive expression site
Explanation
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.
9.
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?
Correct Answer
A. Cytomegalovirus
Explanation
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.
10.
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?
Correct Answer
D. 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.
11.
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?
Correct Answer
A. IgG and complement
Explanation
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.
12.
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?
Correct Answer
C. It induces the synthesis of tumor necrosis factor
Explanation
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.
13.
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?
Correct Answer
C. Rabies virus
Explanation
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.
14.
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?
Correct Answer
A. Gram-positive diplococci that form an a-hemolyticcolony
Explanation
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.
15.
Regarding the function of chemokines in host defenses,
which one of the following is the MOST accurate?
Correct Answer
D. Chemokines attract neutropHils to the site of bacterialinfection, thereby playing a role in the inflammatoryresponse
Explanation
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.
16.
Which one of the following answer choices consists of
bacteria, BOTH of which produce exotoxins that act by
ADP-ribosylation?
Correct Answer
B. Vibrio cholerae and Corynebacterium dipHtheriae
Explanation
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.
17.
Regarding hepatitis C virus (HCV) and hepatitis D virus
(HDV), which one of the following is MOST accurate?
Correct Answer
B. 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.
18.
Which one of the following is MOST likely to induce an
IgM antibody response without the participation of helper
T cells?
Correct Answer
A. Bacterial capsular polysaccharide
Explanation
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.
19.
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?
Correct Answer
D. Listeria monocytogenes
Explanation
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.
20.
Regarding the mode of action of antiviral drugs, which one
of the following is MOST accurate?
Correct Answer
C. Acyclovir action is greater in herpesvirus-infected cellsthan in uninfected cells because herpesvirus-infectedcells contain an enzyme that pHospHorylates acyclovirvery efficiently
Explanation
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.
21.
Which one of the following diseases is MOST likely to be
caused by a delayed hypersensitivity reaction?
Correct Answer
E. Contact dermatitis
Explanation
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.
22.
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?
Correct Answer
D. They have a large amount of lipid in their cell wall that
prevents entry of the crystal violet
Explanation
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.
23.
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?
Correct Answer
E. Mucor species
Explanation
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.
24.
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?
Correct Answer
D. Nonhemolytic colonies on the blood agar plate that
grow in 6.5% sodium chloride
Explanation
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.
25.
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?
Correct Answer
B. Toxoplasma gondii
Explanation
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.
26.
Regarding the diagnosis, treatment, and prevention of HIV,
which one of the following is the MOST accurate?
Correct Answer
C. 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.
27.
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?
Correct Answer
E. Chlamydia trachomatis
Explanation
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.
28.
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?
Correct Answer
C. Nose
Explanation
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.
29.
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?
Correct Answer
D. JC virus
Explanation
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.
30.
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?
Correct Answer
C. Detecting antibodies to cardiolipin in his blood
Explanation
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.
31.
Regarding antimicrobial drugs that act by inhibiting nucleic
acid synthesis in bacteria, which one of the following is the
MOST accurate?
Correct Answer
B. Rifampin inhibits the RNA polymerase in bacteria by
binding to the enzyme and inhibiting messenger RNA
synthesis
Explanation
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.
32.
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?
Correct Answer
D. Red blood cells and albumin in her urine
Explanation
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.
33.
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?
Correct Answer
D. Budding yeasts with a thick capsule
Explanation
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.
34.
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?
Correct Answer
C. 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
Explanation
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.
35.
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?
Correct Answer
D. Neisseria meningitidis
Explanation
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.
36.
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?
Correct Answer
A. Histoplasma capsulatum
Explanation
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.
37.
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?
Correct Answer
B. Propionibacterium sp./Clindamycin
Explanation
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.
38.
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?
Correct Answer
C. Human immunodeficiency virus
Explanation
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.
39.
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?
Correct Answer
D. Pseudomonas aeruginosa
Explanation
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.
40.
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?
Correct Answer
E. Voriconazole
Explanation
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.
41.
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?
Correct Answer
B. Fibrillin
Explanation
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.
42.
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?
Correct Answer
D. Decreased resorption of collagen
Explanation
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.
43.
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?
Correct Answer
C. Increased synthesis of 2,3-bispHospHoglycerate
Explanation
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.
44.
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?
Correct Answer
C. Hydroxylation of proline residues in collagen
Explanation
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.
45.
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?
Correct Answer
D. A misfolded form of a normal protein
Explanation
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.