This practice examination consists of two blocks, each containing 40 microbiology and immunology questions. You should be able to complete each block in 50 minutes. The proportion of the questions devoted to bacteriology, virology, mycology, parasitology, and immunology is approximately that of the USMLE. As in the USMLE, the questions are randomly assorted (i.e., they are not grouped according to subject matter).
All of the questions have between 4 and 10 answer choices. Each question has a single “BEST” answer; there are no “EXCEPT” type questions. The answer choices are listed either in alphabetical order or in order of the length of the answer. The answer key is located at the end of each block.
Directions (Questions 1–40)—Select the ONE lettered answer that is BEST in each question.
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?
(A) Gram-negative rods that grow on EMB agar, forming purple colonies and a green sheen
(B) Gram-positive cocci that grow on blood agar, causing a clear zone of hemolysis, and are coagulase-positive
(C) Gram-positive rods that grow only anaerobically and form a double zone of hemolysis on blood agar
(D) Gram-negative diplococci that grow on chocolate agar, are oxidase-positive, and ferment maltose
(E) Gram-positive cocci that grow on blood agar, causing a green zone of hemolysis, and are not inhibited by optochin and bile
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?
(A) The virion contains a polymerase.
(B) The purified genome RNA is infectious.
(C) The genome RNA is segmented.
(D) A single-stranded DNA is synthesized during replication.
(E) The genome RNA encodes a precursor polypeptide that must be cleaved by a protease.
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?
(A) A deficient amount of the C8 component of complement in his plasma
(B) An inability of his macrophages to present antigen in association with class I MHC proteins
(C) A failure to release granzymes from his cytotoxic T cells
(D) An insufficient amount of IgG in his plasma
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?
(A) Nonencapsulated strains would not have endotoxin.
(B) Nonencapsulated strains cannot secrete exotoxin A.
(C) Nonencapsulated strains should be easily phagocytized.
(D) Nonencapsulated strains should be rapidly killed by ultraviolet light.
(E) Nonencapsulated strains should be susceptible to killing by cytotoxic T cells.
5. A 35-year-old man is HIV antibody positive and has a CD4 count of 50/μL (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?
(A) Aspergillus fumigatus
(B) Cryptococcus neoformans
(C) Mucor species
(D) Histoplasma capsulatum
(E) Coccidioides immitis
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?
(A) Nocardia asteroides
(B) Vibrio vulnificus
(C) Bacteroides fragilis
(D) Haemophilus influenzae
(E) Pseudomonas aeruginosa
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?
(A) Acid-fast rods
(B) Banana-shaped gametocytes
(C) Nonseptate hyphae
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?
(A) It is due to the transposition of existing genes into an active expression site.
(B) It is due to the acquisition of new fertility plasmids by transduction.
(C) It is due to conjugation, during which the recipient obtains new chromosomal genes.
(D) It is due to new mutations that occur at “hot spots” in the genome.
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?
(B) Dengue virus
(C) Hepatitis A virus
(E) Yellow fever virus
10. Which one of the immunoglobulins BEST fits the following description: It is found in plasma as a dimer with a J chain. As it passes through mucosal cells, it acquires a secretory piece that protects it from degradation by proteases.
11. Mycobacterium tuberculosis (MTB) 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?
(A) Cell-mediated immunity is the most important host defense mechanism against MTB, whereas antibody-mediated immunity is the most important host defense mechanism against MAC.
(B) In the clinical laboratory, MAC can be distinguished from MTB by the fact that MAC forms colonies in 7 days, whereas MTB does not.
(C) Multidrug-resistant strains of MAC are much less common than multidrug-resistant strains of MTB.
(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.
12. In the laboratory, a virologist was studying the properties of mutant viruses. When she infected cells with mutant virus #1, no progeny viruses were produced. When she infected cells with mutant virus #2, no progeny viruses were produced. But when she infected cells with both mutant virus #1 and mutant virus #2, progeny viruses of both virus #1 and virus #2 were produced. Which one of the following is the term that BEST describes this phenomenon?
(A) Phenotypic mixing
13. 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?
(A) IgG and complement
(B) IgE and histamine
(C) IL-2 and cytotoxic T cells
(D) Gamma interferon and macrophages
14. 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?
(A) It acts by phosphorylating the G stimulating protein.
(B) It is a polypeptide with an A-B subunit configuration.
(C) It induces the synthesis of tumor necrosis factor.
(D) It is found primarily in gram-positive rods.
(E) It can be treated with formaldehyde to form an effective toxoid vaccine.
15. A 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?
(B) JC virus
(C) Rabies virus
(D) Parvovirus B19
(E) Herpes simplex virus type 1
16. 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?
(A) Gram-positive diplococci that form an α-hemolytic colony
(B) Gram-negative diplococci that form an oxidase-positive colony
(C) Gram-positive rods that form a β-hemolytic colony
(D) Gram-negative rods that form an oxidase-positive colony
(E) Gram-negative cocci that grow only anaerobically
17. Regarding the function of chemokines in host defenses, which one of the following is the MOST accurate?
(A) Chemokines bind to the T-cell receptor outside of the antigen-binding site and activate many T cells.
(B) Chemokines induce gene switching in B cells that increases the amount of IgE synthesized, thereby predisposing to allergies.
(C) Chemokines penetrate the membranes of target cells during attack by cytotoxic T cells.
(D) Chemokines attract neutrophils to the site of bacterial infection, thereby playing a role in the inflammatory response.
18. Which one of the following answer choices consists of bacteria, BOTH of which produce exotoxins that act by ADP-ribosylation?
(A) Salmonella typhi and Vibrio cholerae
(B) Vibrio cholerae and Corynebacterium diphtheriae
(C) Salmonella typhi and Clostridium perfringens
(D) Corynebacterium diphtheriae and Staphylococcus aureus
(E) Clostridium perfringens and Streptococcus pyogenes
19. Regarding hepatitis C virus (HCV) and hepatitis D virus (HDV), which one of the following is MOST accurate?
(A) HCV is transmitted by blood, but HDV is not.
(B) More than half of HCV infections result in a chronic carrier state.
(C) There is an effective vaccine against HCV but not against HDV.
(D) Both HCV and HDV are defective RNA viruses and require concurrent HBV infection to replicate.
20. Which one of the following is MOST likely to induce an IgM antibody response without the participation of helper T cells?
(A) Bacterial capsular polysaccharide
(B) Toxic shock syndrome toxin
(C) Penicillin–bovine serum albumin (BSA) complex
(D) Tetanus toxoid
21. A 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?
(A) Clostridium perfringens
(B) Streptococcus pyogenes
(C) Bacillus cereus
(D) Listeria monocytogenes
(E) Brucella abortus
22. Regarding the mode of action of antiviral drugs, which one of the following is MOST accurate?
(A) Amantadine inhibits influenza A virus by inhibiting the RNA polymerase carried by the virion.
(B) Foscarnet inhibits varicella-zoster virus by inhibiting the RNA polymerase carried by the virion.
(C) Acyclovir action is greater in herpesvirus-infected cells than in uninfected cells because herpesvirus-infected cells contain an enzyme that phosphorylates acyclovir very efficiently.
(D) Azidothymidine inhibits human immunodeficiency virus (HIV) by inhibiting viral mRNA synthesis more efficiently than cellular mRNA synthesis.
(E) Indinavir blocks HIV replication by inhibiting the protease required for the envelope protein gp120 to bind to the CD8 protein on the surface of the T cell.
23. Which one of the following diseases is MOST likely to be caused by a delayed hypersensitivity reaction?
(A) Serum sickness
(B) Poststreptococcal glomerulonephritis
(C) Systemic lupus erythematosus
(D) Hemolytic disease of the newborn
(E) Contact dermatitis
24. 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?
(A) They lack a cell wall; therefore, they cannot adsorb the crystal violet.
(B) They have a very thin cell wall that does not retain the crystal violet.
(C) They have a thick polysaccharide capsule that prevents entry of the iodine solution.
(D) They have a large amount of lipid in their cell wall that prevents entry of the crystal violet.
25. 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?
(A) Candida albicans
(B) Coccidioides immitis
(C) Cryptococcus neoformans
(D) Histoplasma capsulatum
(E) Mucor species
26. 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?
(A) α-Hemolytic colonies on the blood agar plate that are optochin-sensitive
(B) β-Hemolytic colonies on the blood agar plate that are bacitracin-sensitive
(C) β-Hemolytic colonies on the blood agar plate that hydrolyze hippurate
(D) Nonhemolytic colonies on the blood agar plate that grow in 6.5% sodium chloride
27. 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/μL. 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?
(A) Toxocara canis
(B) Toxoplasma gondii
(C) Taenia saginata
(D) Trichinella spiralis
(E) Trypanosoma cruzi
28. The emergence of antibiotic-resistant bacteria, especially in enteric gram-negative rods, is an extremely important phenomenon. The acquisition of resistance most commonly occurs by a process that involves a sex pilus and the subsequent transfer of plasmids carrying one or more transposons. Which one of the following is the name that BEST describes this process?
29. Regarding the diagnosis, treatment, and prevention of HIV, which one of the following is the MOST accurate?
(A) The drug zidovudine (AZT) is a “chain terminating” drug; that is, it inhibits the growing polypeptide chain by causing misreading of the viral mRNA.
(B) The drug lamivudine (3TC) acts by binding to the integrase, which prevents integration of the viral DNA into cellular DNA.
(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.
(D) A major limitation to our ability to produce a vaccine against HIV is that there are many serologic types of the viral p24 protein.
30. Regarding haptens, which one of the following statements is the MOST accurate?
(A) They are typically polypeptides that are resistant to proteolytic cleavage within the antigen-presenting cell.
(B) They bind to class II MHC proteins but not to class I MHC proteins.
(C) They cannot induce antibodies unless they are bound to a carrier protein.
(D) They activate complement by binding to the Fc part of the heavy chain of IgG.
31. 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?
(A) Treponema pallidum
(B) Haemophilus ducreyi
(C) Mycobacterium marinum
(D) Candida albicans
(E) Chlamydia trachomatis
32. Regarding host defenses against viruses, which one of the following is MOST accurate?
(A) IgA exerts its main antiviral effect by enhancing the cytopathic effect of natural killer cells—a process called antibody-dependent cellular cytotoxicity.
(B) IgG plays a major role in neutralizing virus infectivity during the primary infection.
(C) Complexes of virus and IgE are the cause of the inflammatory arthritis seen in several viral infections, such as hepatitis B and rubella.
(D) Alpha and beta interferons exert their antiviral action by inducing a ribonuclease that degrades viral mRNA and a protein kinase that inactivates protein synthesis.
(E) Alpha and beta interferons exert their antiviral effect against viruses with RNA genomes but not against those with DNA genomes.
33. Allergic rhinitis is characterized by sneezing, rhinorrhea, nasal congestion, and itching of the eyes and nose. Persons with allergic rhinitis have “X” that binds to high-affinity receptors on “Y.” On reexposure to antigen, the “Y” of patients with allergic rhinitis degranulate, releasing “Z” and other mediators. Which one of the following sets BEST describes X, Y, and Z?
(A) X is IgE, Y is macrophages, and Z is tumor necrosis factor.
(B) X is IgE, Y is basophils, and Z is histamine.
(C) X is IgG, Y is eosinophils, and Z is histamine.
(D) X is IgG, Y is neutrophils, and Z is tumor necrosis factor.
(E) X is IgA, Y is eosinophils, and Z is interleukin-5.
34. 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?
(B) Gingival crevice
35. 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?
(C) Herpes simplex virus
(D) JC virus
(E) Coxsackie virus
36. 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?
(A) Blood culture
(B) Culture on Thayer-Martin medium (chocolate agar with antibiotics)
(C) Detecting antibodies to cardiolipin in his blood
(D) Detecting treponemal antigen in his blood
(E) Western blot assay
37. A 22-year-old woman has an erythematous rash on the malar eminences of her face that gets worse when she is out in the sun. She has lost about 10 lb and feels tired much of the time. She took her temperature a few times, and it was 99°F. Physical examination was normal except for the rash. Laboratory tests revealed a hemoglobin of 11 and a white blood cell count of 5500. Urinalysis showed albumin in the urine but no red cells, white cells, or bacteria. Which one of the following is the MOST likely laboratory finding in this disease?
(A) Decreased number of helper (CD4-positive) T cells
(B) High level of antibodies to double-stranded DNA
(C) Increased number of cytotoxic (CD8-positive) T cells
(D) Low level of C1 inhibitor
(E) Low microbicidal activity of neutrophils
38. Regarding antimicrobial drugs that act by inhibiting nucleic acid synthesis in bacteria, which one of the following is the MOST accurate?
(A) Quinolones, such as ciprofloxacin, inhibit the RNA polymerase in bacteria by acting as nucleic acid analogues.
(B) Rifampin inhibits the RNA polymerase in bacteria by binding to the enzyme and inhibiting messenger RNA synthesis.
(C) Sulfonamides inhibit the DNA polymerase in bacteria by causing chain termination of the elongating strand.
(D) Trimethoprim inhibits the DNA polymerase in bacteria by preventing the unwinding of double-stranded DNA.
39. Regarding parvovirus B19, which one of the following is the MOST accurate?
(A) Parvovirus B19 has a double-stranded DNA genome but requires a DNA polymerase in the virion because it replicates in the cytoplasm.
(B) Parvovirus B19 is transmitted primarily by sexual intercourse.
(C) Parvovirus B19 causes severe anemia because it preferentially infects erythrocyte precursors.
(D) Patients infected by parvovirus B19 can be diagnosed in the laboratory using the cold agglutinin test
(E) Patients with disseminated disease caused by parvovirus B19 should be treated with acyclovir.
40. Which one of the following laboratory tests would be the BEST to order to determine the number of CD4-positive cells in a patient infected with HIV?
(B) Enzyme-linked immunosorbent assay (ELISA)
(C) Flow cytometry
(E) Ouchterlony gel assay
ANSWERS TO BLOCK ONE
1. 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?
(A) Bloody diarrhea
(B) Blurred vision
(C) Paralysis of the facial nerve (Bell’s palsy)
(D) Red blood cells and albumin in her urine
(E) Rusty-colored sputum
2. The purified genome of certain RNA viruses can enter a cell and elicit the production of progeny viruses (i.e., the genome is infectious). Regarding these viruses, which one of the following statements is MOST accurate?
(A) They have a segmented genome.
(B) They have a polymerase in the virion.
(C) Their genome RNA is double-stranded.
(D) They encode a protease that cleaves a precursor polypeptide.
(E) Their genome RNA has the same base sequence as mRNA.
3. A 77-year-old man with enterococcal endocarditis needed to be treated with penicillin G but had a history of a severe penicillin reaction. He was therefore skin tested using penicilloyl-polylysine as the antigen. Which one of the following is MOST likely to occur in a positive skin test?
(A) The antigen forms immune complexes with IgG.
(B) The antigen activates CD4-positive T cells and macrophages.
(C) The antigen activates the alternative pathway of complement.
(D) The antigen activates CD8-positive T cells by binding to class I MHC proteins.
(E) The antigen cross-links IgE on the mast cells and causes the release of histamine.
4. Regarding the Gram stain, which one of the following is the MOST accurate?
(A) After adding crystal violet and Gram’s iodine, both gram-positive bacteria and gram-negative bacteria will appear blue.
(B) If you forget to stain with the red dye (safranin or basic fuchsin), both gram-positive bacteria and gram-negative bacteria will appear blue.
(C) If you forget to heat-fix, both gram-positive bacteria and gram-negative bacteria will appear blue.
(D) One reason why bacteria have a different color in this stain is because the gram-positive bacteria have lipid in their membrane, whereas gram-negative bacteria do not.
5. 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?
(B) Non-septate hyphae
(C) Germ tubes
(D) Budding yeasts with a thick capsule
(E) Septate hyphae with low-angle branching
6. A 22-year-old woman complains of a persistent nonproductive cough and a fever of 101°F that came on slowly over the last 4 days. Physical examination reveals some rales in the left lung base. A patchy infiltrate is seen on chest X-ray. She works as a secretary in a law office and has not traveled recently. She is not immunocompromised and has not been hospitalized recently. A sample of her serum agglutinates red blood cells at 4°C but not at 37°C. Which one of the following BEST describes the organism that is the MOST likely cause of her disease?
(A) A very small bacterium that has no cell wall
(B) A gram-negative diplococcus with a large capsule
(C) An acid-fast rod that forms colonies within 7 days
(D) A filamentous gram-positive rod that is weakly acid-fast
(E) A spirochete that has never been grown on blood agar
7. The mother of a 4-year-old child notes that her child is sleeping poorly and scratching his anal area. You suspect the child may have pinworms. Which one of the following is the BEST method to make that diagnosis?
(A) Examine the stool for the presence of cysts
(B) Examine the stool for the presence of trophozoites
(C) Examine a blood smear for the presence of microfilaria
(D) Determine the titer of IgE antibody against the organism
(E) Examine transparent adhesive tape for the presence of eggs
8. Regarding bacterial spores, which one of the following is the MOST accurate?
(A) One spore germinates to form one bacterium.
(B) They are produced primarily within human red blood cells.
(C) They are killed by boiling at sea level but not at high altitude.
(D) They are produced by anaerobes only in the presence of oxygen.
(E) They contain endotoxin, which accounts for their ability to cause disease.
9. 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: HAV-IgM negative, HAV-IgG positive, HBsAg negative, HBsAb positive, HBcAb negative, HCV-Ab positive. Of the following, which one is the MOST accurate?
(A) She probably has hepatitis A now, probably has not been infected with hepatitis B virus (HBV), and probably had hepatitis C in the past.
(B) She probably has hepatitis A now, probably has been infected with HBV in the past, and probably had hepatitis C in the past.
(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.
(D) She has been infected with HAV in the past, probably has hepatitis B now, and probably had hepatitis C in the past.
10. Regarding the function of the different classes of antibodies, which one of the following statements is the MOST accurate?
(A) IgA acts as an antigen receptor on the surface of B cells.
(B) IgG activates the alternative pathway of complement, resulting in the production of C3a that degrades the bacterial cell wall.
(C) IgG binds to the bacterial surface and makes the bacteria more easily ingested by phagocytes.
(D) IgM defends against worm parasites, such as hookworms.
(E) IgE blocks the binding of viruses to the gut mucosa.
11. A 6-year-old boy fell and sustained a deep wound from a rusty nail that penetrated his thigh. His mother removed the nail and cleaned the wound with soap and water. The next morning, he had a temperature of 102°F, and his thigh was very painful and swollen. In the emergency room, crepitus (gas in the tissue) was noted. A Gram stain of exudate from the wound area revealed large gram-positive rods. Which one of the following is the MOST likely cause?
(A) Actinomyces israelii
(B) Clostridium perfringens
(C) Clostridium tetani
(D) Listeria monocytogenes
(E) Mycobacterium fortuitum-chelonei complex
(F) Nocardia asteroides
(G) Pseudomonas aeruginosa
12. The two most common types of viral vaccines are killed vaccines and live, attenuated vaccines. Regarding these vaccines, which one of the following statements is the MOST accurate?
(A) Killed vaccines induce a longer-lasting response than do live, attenuated vaccines.
(B) Killed vaccines are no longer used in this country because they do not induce secretory IgA.
(C) Killed vaccines induce a broader range of immune responses than do live, attenuated vaccines.
(D) Killed vaccines are safer to give to immunocompromised patients than are live, attenuated vaccines.
13. Regarding anaphylactic (type I) and immune complex (type III) hypersensitivities, which one of the following is the MOST accurate?
(A) IgE is involved in both anaphylactic and immune complex hypersensitivities.
(B) Complement is involved in both anaphylactic and immune complex hypersensitivities.
(C) Less antigen is typically needed to trigger an anaphylactic reaction than an immune complex reaction.
(D) Neutrophils play a more important role in anaphylactic reactions than in immune complex reactions.
14. Disease caused by which one of the following bacteria can be prevented by a toxoid vaccine?
(A) Actinomyces israelii
(B) Bacteroides fragilis
(C) Borrelia burgdorferi
(D) Corynebacterium diphtheriae
(E) Haemophilus influenzae
(F) Listeria monocytogenes
(G) Neisseria meningitidis
(H) Salmonella typhi
(I) Streptococcus pneumoniae
(J) Yersinia pestis
15. A 50-year-old woman has had a gradual onset of headaches that have become increasingly more severe during the past 3 weeks. On examination, she is confused regarding time, place, and person, and she is febrile to 39°C. Her spinal fluid reveals a normal glucose, normal protein, and 17 cells, all of which were lymphocytes. Gram stain of the spinal fluid shows no organism. An MRI reveals a 2-cm radiolucent lesion in the temporal lobe. A biopsy of the brain lesion was performed. A Giemsa stain of the tissue shows multinucleated giant cells with intranuclear inclusion bodies. Which one of the following is the MOST likely causative organism?
(B) Coxsackie virus
(D) Herpes simplex virus type 1
(E) Influenza virus
(F) Measles virus
(G) Parvovirus B19
(J) Rabies virus
16. An 80-year-old man had a carcinoma of the colon removed 3 days ago. He was doing well until this morning, when he spiked a fever to 39°C and complained of severe abdominal pain. Examination revealed a “board-like” abdomen indicative of peritonitis. He was taken to the operating room, where it was discovered that his anastomosis had broken down and bowel contents had spilled into the peritoneal cavity. A foul-smelling exudate was observed. A Gram stain of the peritoneal exudate revealed many gram-negative rods. Which one of the following sets of bacteria is the MOST likely cause of this infection?
(A) Escherichia coli and Brucella melitensis
(B) Enterobacter cloacae and Salmonella enteritidis
(C) Fusobacterium nucleatum and Bacteroides fragilis
(D) Haemophilus influenzae and Actinomyces israelii
(E) Shigella dysenteriae and Serratia marcescens
17. Regarding the primary and secondary antibody responses, which one of the following statements is MOST accurate?
(A) The IgM made in the primary response is made primarily by memory B cells.
(B) The lag phase is shorter in the primary response than in the secondary response.
(C) In the primary response, memory B cells are produced, but memory T cells are not.
(D) Antigen must be processed and presented in the primary response but not in the secondary response.
(E) The amount of IgG made in the secondary response is greater than the amount made in the primary response.
18. A 70-year-old man who is receiving chemotherapy for leukemia develops a fever to 40°C and has two episodes of teeth-chattering chills, and his blood pressure drops to 80/20 mmHg. Of the following factors, which one is MOST likely to be the cause of his fever, chills, and hypotension?
(B) Dipicolinic acid
(D) Lipid A
(E) Mycolic acid
(G) Polysaccharide capsule
19. A 22-year-old woman presents with “the worst sore throat I’ve ever had.” She also complains of fatigue and anorexia. She is not immunocompromised and has not been hospitalized recently. On examination, she is febrile to 38°C, the pharynx is inflamed, and there are a few tender cervical nodes bilaterally. There are no white lesions on the tongue or pharynx. A throat culture grows α-hemolytic colonies on blood agar that are optochin-resistant. Of the following, which one is the MOST likely cause?
(A) Candida albicans
(B) Epstein–Barr virus
(C) Parvovirus B19
(D) Pneumocystis carinii
(F) Serratia marcescens
(G) Streptococcus mutans
(H) Streptococcus pneumoniae
(I) Streptococcus pyogenes
(J) Strongyloides stercoralis
20. Regarding the complement pathway, which one of the following is MOST accurate?
(A) C5a mediates chemotaxis and attracts neutrophils to the site of infection.
(B) C5b plays an important role in the opsonization of gram-negative bacteria.
(C) C3a is a decay-accelerating factor that causes the rapid decay and death of bacteria.
(D) C1 binds to the surface of gram-positive bacteria, which initiates the classic pathway.
(E) The membrane attack complex is produced in the classic path-way but not in the alternative pathway.
21. A 65-year-old woman had symptoms of dementia. An MRI revealed significant cortical atrophy. It was determined that her intraventricular pressure was very high, and a ventriculoperitoneal shunt (from the brain, tunneling under the skin into the peritoneal cavity) was placed to relieve the pressure. Three weeks later, she developed a fever to 38°C, malaise, and anorexia but no other symptoms. Of the following, which one BEST describes the MOST likely organism causing her current symptoms?
(A) A gram-positive coccus that does not clot plasma
(B) A curved gram-negative rod that produces urease
(C) An acid-fast rod that does not grow on bacteriologic media
(D) An obligate intracellular parasite that forms a cytoplasmic inclusion body
(E) A spirochete that induces an antibody that agglutinates a lipid from a cow’s heart
22. Two mutants of poliovirus, one mutated at gene X and the other mutated at gene Y, have been isolated. If a cell is infected with each mutant alone, no virus is produced. If a cell is infected with both mutants, which one of the following is MOST likely to occur?
(A) Complementation between the mutant gene products may occur, and, if so, both X and Y progeny viruses will be made.
(B) Phenotypic mixing may occur, and, if so, both X and Y progeny viruses will be made.
(C) Reassortment of the genome segments may occur, and, if so, both X and Y progeny viruses will be made.
(D) The genome may be transcribed into DNA, and, if so, both X and Y viruses will be made.
23. A 40-year-old woman has a history of chronic inflammation of the small joints of the hands bilaterally. You suspect rheumatoid arthritis. Which one of the following statements is the MOST accurate regarding the pathogenesis of this disease?
(A) It is caused by sensitized CD4-positive T lymphocytes and macrophages invading the joints.
(B) It is caused by antibody against human IgG-forming immune complexes within the joints.
(C) It is caused by the release of mediators from mast cells when environmental agents cross-link adjacent IgEs within the joints.
(E) It is caused by superantigens inducing the release of large amounts of lymphokines from helper T cells within the joints.
24. Listed below are five bacteria paired with a mode of transmission. Which one of the pairings is MOST accurate?
(A) Borrelia burgdorferi—mosquito bite
(B) Coxiella burnetii—bat guano
(C) Haemophilus influenzae—penetrating wound contaminated with soil
(D) Rickettsia rickettsii—contaminated food
(E) Yersinia pestis—flea bite
25. A 70-year-old man with leukemia initially responded to chemotherapy but now is refractory. He therefore underwent a bone marrow transplant and is now receiving large doses of cyclosporine A and prednisone. Three weeks after the transplant, he became febrile to 39°C and began coughing up purulent sputum. A chest X-ray revealed pneumonia. A Gram stain of the sputum did not reveal a predominant organism, but a KOH prep of the sputum revealed septate hyphae with parallel walls and low-angle branching. Of the following organisms, which one is MOST likely to be the cause of this pneumonia?
(A) Aspergillus fumigatus
(B) Candida albicans
(C) Coccidioides immitis
(D) Cryptococcus neoformans
(E) Rhizopus nigricans
26. Your patient is a 20-year-old woman with severe diarrhea that began yesterday. She has just returned from a 3-week trip to Peru, where she ate some raw shellfish at the farewell party. She now has watery diarrhea, perhaps 20 bowel movements a day, and is feeling quite weak and dizzy. Her stool is guaiac-negative, a test that determines whether there is blood in the stool. A Gram stain of the stool reveals curved gram-negative rods. Of the following organisms, which one is MOST likely to be the cause of her diarrhea?
(A) Bacteroides fragilis
(B) Campylobacter jejuni
(C) Entamoeba histolytica
(D) Helicobacter pylori
(E) Shigella dysenteriae
(F) Vibrio cholerae
(G) Yersinia enterocolitica
27. A 50-year-old man has had low-grade, persistent headaches for several months. In the last few days, nausea, vomiting, and blurred vision have occurred. An MRI reveals several cystlike lesions in the brain parenchyma. The patient lived for many years on one of the small Caribbean islands. On the basis of a positive serologic test, a diagnosis of neurocysticercosis was made. Of the following, which one is the MOST likely mode by which this disease was acquired?
(A) Sandfly bite
(B) Mosquito bite
(C) Sexual intercourse
(D) Ingestion of the larvae of the organism in raw fish
(E) Ingestion of the eggs of the organism in contaminated food
(F) Penetration of the skin by the organism while walking bare-footed
(G) Penetration of the skin by the organism while bathing in fresh water
28. A 30-year-old woman with a previous history of rheumatic fever now has a fever for the past 2 weeks. Physical examination reveals a new heart murmur. You suspect endocarditis and do a blood culture, which grows a viridans group streptococcus later identified as Streptococcus sanguis. Of the following body sites, which one is the MOST likely source of this organism?
29. Regarding poliovirus, which one of the following is MOST accurate?
(A) Poliovirus remains latent within sensory ganglia, and reactivation occurs primarily in immunocompromised patients.
(B) When the live, attenuated virus in the oral vaccine replicates, revertant mutants can occur that can cause paralytic polio.
(C) The widespread use of the killed vaccine in the countries of North and South America has led to the virtual elimination of paralytic polio in those areas.
(D) The current recommendation is to give the live, attenuated vaccine for the first 3 immunizations to prevent the child from acting as a reservoir, followed by boosters using the killed vaccine.
30. Regarding ABO and Rh blood types, which one of the following is the MOST accurate?
(A) People with type O are called universal recipients because they have antibodies against H substance but not against A and B antigens.
(B) If the father is Rh-positive and the mother is Rh-negative, hemolytic disease of the newborn only occurs when the child is Rh-negative.
(C) People who are Rh-negative usually have antibodies to the Rh antigen because they are exposed to cross-reacting antigen located on bacteria in the colon.
(D) If type A blood is transfused into a person with type B blood, complement will be activated, and the membrane attack complex will cause lysis of the type A red cells.
31. A 25-year-old man was in a motorcycle accident 3 days ago, in which he sustained severe head trauma. He has had spinal fluid leaking from his nose since the accident and now develops a severe headache. His temperature is 39°C, and on examination you find nuchal rigidity. You do a lumbar puncture and find that the spinal fluid is cloudy and contains 5000 WBC/μL, 90% of which are polys. Of the following, which one is the MOST likely result observed in the laboratory analysis of the spinal fluid?
(A) Gram-negative rods that grew only anaerobically
(B) A motile spirochete that formed β-hemolytic colonies on blood agar
(C) Gram-positive cocci that formed α-hemolytic colonies on blood agar
(D) Gram-positive cocci that grew only in the presence of 6.5% sodium chloride
(E) Gram-positive rods that grew only on chocolate agar supplemented with X and V factors
(F) No organism was seen using Gram stain, but tissue stains revealed cytoplasmic inclusion bodies
32. Regarding prions and prion-caused diseases, which one of the following is MOST accurate?
(A) Prions are highly resistant to both ultraviolet light and to boiling but are inactivated by hypochlorite.
(B) Prions are protein-containing particles surrounded by a lipoprotein envelope with a DNA polymerase in the envelope.
(C) The diagnosis of prion-caused diseases such as Creutzfeldt-Jakob disease is typically made by observing cytopathic effect in cell culture.
(D) Creutzfeldt-Jakob disease occurs primarily in children younger than the age of 2 years because they cannot mount an adequate immune response to the prion protein.
33. A 2-year-old boy has had several infections of the sinuses and lungs and is being evaluated to determine whether he has chronic granulomatous disease. Regarding this disease, which one of the following is the MOST accurate?
(A) There is a deficiency in NADPH oxidase activity.
(B) The defect is primarily in antigen-presenting cells such as macrophages.
(C) Pneumocystis carinii infections are common in patients with this disease.
(D) The diagnosis is primarily made by ELISA, in which antibody against the affected cell component is detected.
34. Regarding Chlamydiae, which one of the following is MOST accurate?
(A) They are gram-positive rods that do not form spores.
(B) They exhibit swarming motility on a blood agar plate.
(C) Their life cycle consists of a metabolically inactive particle in the extracellular phase.
(D) They can replicate only within cells because they lack the ability to produce certain essential mRNAs.
(E) They replicate in the nucleus of infected cells, where they form inclusions that are useful diagnostically.
35. Regarding human papillomavirus (HPV), which one of the following is MOST accurate?
(A) Blood and blood products are an important mode of transmission of HPV.
(B) HPV is an enveloped virus with a genome composed of double-stranded RNA.
(C) Amantadine is a chain-terminating drug that inhibits HPV replication by blocking DNA synthesis.
(D) HPV induces the formation of koilocytes in the skin that are an important diagnostic feature of HPV infection.
(E) The P2 capsid protein of HPV activates the c-sarc oncogene in human cells, which is the process by which HPV predisposes to malignancy
36. Regarding Lyme disease, which one of the following is MOST accurate?
(A) The causative organism is a small gram-positive rod.
(B) Mice are the main reservoir of the causative organism.
(C) The Lyme disease vaccine contains toxoid as the immunogen.
(D) Fleas are the principal mode of transmission of the causative organism.
(E) The diagnosis in the clinical laboratory is typically made by culturing the organism on chocolate agar.
37. Regarding Bruton’s agammaglobulinemia, which one of the following is the MOST accurate?
(A) VDJ gene switching does not occur.
(B) There is very little IgG, but IgM and IgA levels are normal.
(C) The number of B cells is normal, but they cannot differentiate into plasma cells.
(D) There is a defect in a tyrosine kinase, one of the enzymes in the signal transduction pathway.
(E) Viral infections are more common in patients with this disease than are pyogenic bacterial infections.
38. A 20-year-old woman presents with a history of vaginal discharge for the past 3 days. On pelvic examination, you see a mucopurulent exudate at the cervical os, and there is tenderness on palpation of the right fallopian tube. You do a Gram stain and culture on the cervical discharge. The culture is done on Thayer-Martin medium, which is a chocolate agar that contains antibiotics that inhibit the growth of normal flora. Of the following, which findings are the MOST likely to be found?
(A) A Gram stain reveals many neutrophils and spirochetes, and culture on Thayer-Martin medium reveals no colonies.
(B) A Gram stain reveals many neutrophils and gram-variable rods, and culture on Thayer-Martin medium reveals β-hemolytic colonies.
(C) A Gram stain reveals many neutrophils and gram-negative diplococci, and culture on Thayer-Martin medium reveals oxidase-positive colonies.
(D) A Gram stain reveals many neutrophils but no gram-negative diplococci are seen, and culture on Thayer-Martin medium reveals coagulase-positive colonies.
39. Regarding human immunodeficiency virus (HIV), which one of the following is MOST accurate?
(A) The term viral load refers to the concentration of HIV RNA in the patient’s blood plasma.
(B) Both zidovudine and lamivudine block HIV replication by inhibiting cleavage of the precursor polypeptide by the virion-encoded protease.
(C) The antigenicity of the GAG protein of HIV is highly variable, which is a significant impediment to the development of a vaccine against HIV.
(D) The Western blot test for antibodies to HIV has more false-positive results than the ELISA test.
40. Regarding Th-1 and Th-2 cells, which one of the following is MOST accurate?
(A) Th-1 cells produce gamma interferon and promote cell-mediated immunity.
(B) Th-2 cells produce interleukin-12, which inhibits the formation of Th-1 cells.
(C) Both Th-1 and Th-2 cells have class II MHC proteins on their outer cell membrane.
(D) Before they differentiate into Th-1 or Th-2 cells, naïve Th cells are double-positives (i.e., they produce both gamma interferon and interleukin-4).
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