USMLE Step1 Microbiology

1. _____ A 7-year-old patient presents with pain in the left ear. Pus is drained from the ear and a sample sent for culture. Gram stain reveals neutrophils and small Gram-negative bacteria. A common pathogen in this setting is Haemophilus influenzae. However, the bacteria in the smear are so small that it is difficult to decide whether they are really rods or cocci. What information from the lab would allow identification of the organism as H. influenzae?

A. Growth on chocolate agar in elevated CO2 but not room air.
B. β-hemolysis around colonies on sheep blood agar.
C. Ability to grow in medium containing 6.5% NaCl.
D. Sensitivity to bacitracin and resistance to optochin.
E. Positive bile-esculin test.
F. Growth on broth-based agar supplemented with hemin and NAD
(= Factors X and V).

2. _____ Successful immunization against Tetanus results in production of antibodies that prevent disease.
By what mechanism do these protective antibodies prevent Tetanus?

A. Opsonize cells of C. tetani.
B. Prevent binding of C. tetani endotoxin to monocyte receptors.
C. Bind C. tetani flagella, prevent cell motility and access to neurons.
D. Bind C. tetani cells and activate complement.
E. Bind toxin receptors on neuron plasma membranes, prevent binding/entry of toxin.
F. Neutralize the protein exotoxin of C. tetani.

3. _____ A 15-year-old male from west-central New Jersey has been ill for five days with fever, chills, and headache, and today woke to find a rash covering his body and limbs, including his palms and soles.
The rash is composed of fine macular lesions (macule = flat spot, red but not raised). Rocky Mountain
Spotted Fever should be high on your list of possible causes. To help confirm or rule out this possibility, you need to ask him about possible exposure to the source of this disease.
What should you ask him about?

A. Drinking un-chlorinated water.
B. Eating raw meat.
C. Unprotected sex.
D. Tick bites.
E. Contact with pet reptiles or birds.

Questions 4-6: An elderly man with a post-surgical infection.

An intravenous catheter was put in place for delivery of antibiotics and other medications. He developed fever and pneumonia 3 days after surgery. Sputum culture on sheep blood agar grew out numerous small gray α- hemolytic colonies; about 10% of colonies were larger, white, shiny, opaque, and β-hemolytic. Only the latter type grew from blood cultures. Both colony types contained Gram-positive cocci. The lab reported the small colonies as ‘oral streptococci’ and the larger colonies as ‘coagulase-negative Staphylococci’.

4. _____ What test was used to distinguish Staphylococci from Streptococci?

A. Catalase test
B. Coagulase test
C. Oxidase test
D. Sensitivity to optochin
E. Sensitivity to bacitracin
F. CAMP test

5. _____ What does the coagulase-negative result indicate?

A. The organism is more virulent than other staphylococci.
B. The organism is likely to produce a superantigen toxin.
C. The patient is in little or no danger of developing septic shock.
D. The Gram-positive staining was likely the result of improper laboratory procedure.
E. The catheter entry site is likely to have been infected by normal skin flora.

6. _____ An important virulence factor of coagulase-positive staphylococci is Protein A.
How does Protein A aid in virulence?

A. Binds the Fc region of IgG, decreases opsonization.
B. Hydrolyzes secretory IgA.
C. Binds Factor H, prevents activation of complement.
D. Extracts iron from plasma proteins.
E. Promotes tight binding of bacteria to extracellular matrix.

Questions 7-9: An HIV-positive patient with hemoptysis.

He is 30 years old and has been referred to you because of hemoptysis (= coughing up blood). A cavitary lesion in the right upper lobe of the lung is seen on chest x-ray (Cavitary lesion = opaque lesion on x-ray with a central region of low density produced by death of tissue.) A sample of sputum is obtained and a smear is acid-fast-stained using the red dye carbol-fuchsin.

7. _____ If this is tuberculosis, what would you expect to observe after acid-fast stain of sputum?

A. No stained bacteria
B. Irregularly-staining red branching filaments.
C. Red acid-fast cocci, in grape-like clusters
D. Blue-staining bacilli.
E. Red-staining bacilli.

8. _____ The patient is given a tuberculin test. Three days later, there is induration and erythema, 32 mm in diameter, at the site of injection, a positive test result.
What process produced the induration and erythema?

A. Activation of complement.
B. Induction of a primary antibody response.
C. Invasion and lysis of epithelial cells by bacteria.
D. Cytokine production by CD4+ T lymphocytes.
E. Release of inflammatory cell-envelope components from bacteria.

9.______ The sputum sample was sent for culture on Lowenstein-Jensen agar and in the BACTEC system.
The probable diagnosis was readily established by x-ray, direct smear, and skin testing.
Why was the labor- and time-intensive culture procedure performed?

A. Many fungi stain acid-fast.
B. Only highly-virulent mycobacteria grow on the media used for culture.
C. Culture allows testing for antibiotic sensitivity/resistance.
D. Rapid growth of mycobacteria correlates with high virulence.
Choose the single best answer to each question.

10. _____ Streptococcus pneumoniae, Haemophilus influenzae, Neisseria meningitidis, and Cryptococcus neoformans all colonize the upper respiratory tract and can spread from there to the meninges. What essential virulence factor, that aids meningeal spread, is produced by all four organisms?

A. IgA protease
B. Polysaccharide capsule
C. Thick peptidoglycan
D. Endotoxin
E. Exotoxin

11. _____ Seven children in a third-grade class developed bloody diarrhea; one of them was hospitalized with renal failure. Stool cultures for Gram-positive and Gram-negative bacteria (including Campylobacter) yielded only “normal intestinal flora” and “lactose-positive organisms” on MacConkey agar. Epidemiologists found that, three days before the outbreak, the class had gone to a local restaurant for a party. All children who became ill, and none who did not, had eaten hamburgers. Later it was found that some lactose fermenters failed to ferment sorbitol. Which organism is most strongly suggested by the laboratory and epidemiologic data?

A. Bacillus cereus
B. Campylobacter jejuni
C. Clostridium perfringens
D. Escherichia coli
E. Listeria monocytogenes
F. Salmonella enterica
G. Staphylococcus aureus
H. Yersinia enterocolitica

12._____ An 18-year-old man is seen in clinic for urethral discharge. Gram-stained smear of exudate contains neutrophils and Gram-negative diplococci, many of them intracellular. He is treated with ceftriaxone (a β-lactam) but does not get better, and returns. On his second visit, Gram-stained smear of exudate contains neutrophils but no bacteria are visible, and culture of exudate produces no colonies on Sheep blood, Chocolate, or Thayer-Martin agars in elevated CO2.
Which two species below are most likely to have caused his infections?

A. Klebsiella pneumoniae and Chlamydia pneumoniae
B. Neisseria meningitidis and Chlamydia trachomatis
C. Legionella pneumophila and Mycoplasma pneumoniae
D. Treponema pallidum and Ehrlichia phagocytophila
E. Neisseria gonorrhoeae and Mycoplasma hominis

Questions 13-15: A young woman with a post-chemotherapy infection.

A 21-year-old woman with leukemia was treated with a brief intensive course of cytotoxic drugs. Ten days after therapy began she developed fever and shortness of breath. A CBC (complete blood count) showed her to be highly neutropenic. Chest x-ray revealed a dense lesion in her left lung. Sputum culture grew out “normal oral flora” plus large fuzzy colonies. Under the microscope the fuzzy colonies contained hyphae (with septa) which branched at acute angles, and conidia in chains. In a silver-stained biopsy of the lung lesion branching hyphae were seen in tissue.

13. _____ What process is most likely to have created the patient’s neutropenia?

A. Cytokine production by leukemic cells.
B. Killing of dividing cells by cytotoxic anti-cancer drugs.
C. Killing by toxins secreted by the pathogen.
D. Induction of apoptosis by the pathogen.
E. A genetic defect in neutrophil production.

14. _____ To what genus is the pathogen most likely to belong?

A. Actinomyces
B. Aspergillus
C. Klebsiella
D. Mycobacterium
E. Mycoplasma
F. Mucor
G. Pneumocystis
H. Staphylococcus
I. Streptococcus
J. Rhizopus

15. _____ With what drug should the patient be treated?

A. Another β-lactam
B. A β-lactam plus an aminoglycoside
C. An aminoglycoside alone
D. An azole
E. A tetracycline

16. _____ Gastroscopy reveals that a patient with stomach pain has a large ulceration of his gastric mucosa. During the procedure a biopsy of the stomach epithelium is obtained.
Which observation would suggest Helicobacter pylori as the cause of illness?

A. Curved or spiral bacteria seen in the biopsy after silver staining.
B. Biopsy shows infiltration of gastric epithelium by lymphocytes.
C. pH of stomach fluid is equal to 1 (normal), rather than decreased.
D. Ingested isotope-labeled nitrate is converted to nitrite in the stomach.
E. Bacteria are found in stomach fluid, but not in contact with the gastric epithelium.

17. _____ A 75-year-old man reports malaise, headache, and fever. On examination his neck is mildly stiff. Gram stain of the patient's CSF reveals neutrophils and numerous Gram-negative cocci, many in pairs. Which organism below is most likely?

        Pseudomonas aeruginosa
        Klebsiella pneumoniae
        Haemophilus influenzae
        Escherichia coli
        Treponema pallidum
        Nocardia asteroides
        Neisseria gonorrhoeae
        Neisseria meningitides

18. _____ A young man develops peritonitis following abdominal trauma. During surgery foul-smelling purulent material is aspirated from the infected area. Gram stain of this material contains a mixture of Gram positive cocci, Gram-positive rods, and many Gram-negative rods. Aerobic culture on sheep blood agar yields many colonies of enterococci and a few colonies of E. coli; culture on MacConkey agar produces only sparse colonies of E. coli. Anaerobic culture on sheep blood agar produces a few colonies of β-hemolytic Gram-positive rods and many colonies of slender Gram-negative rods.
Which organism is most likely to represent the majority of the Gram-negative rods seen when peritoneal exudate was stained?

        Yersinia enterocolitica
        Clostridium perfringens
        Clostridium tetani
        Pseudomonas aeruginosa
        Salmonella typhi
        Yersinia pseudotuberculosis
        Bacteroides fragilis

Questions 19-20: A woman with neurological illness.

19. _____ A 55-year-old woman develops diplopia, one day after eating home-canned fruit. (Diplopia = double vision, from inability to coordinate the direction of gaze of the eyes.) A few hours later she develops difficulty speaking, bilateral weakness of her arms, nausea, vomiting, dizziness, blurred vision, and dry mouth. However she is alert, oriented, and has no fever.
If this is botulism, by what mechanism did Clostridium botulinum produce her signs and symptoms?

A. Bound to Toll-like receptors and stimulated production of inflammatory cytokines.
B. Injected proteins into neurons via a type III secretion system.
C. Secreted a protein toxin that enters mammalian neurons and blocks synaptic transmission.
D. Released peptidoglycan and other inflammatory components of the cell envelope.
E. Bacteremic spread followed by infection of multiple tissues: eyes, muscle, stomach, nervous system, salivary glands.

20. _____ Home-canned food is a common source of botulism in adults. What property of C. botulinum facilitates infection by this route?

A. Produces airborne spores not easily killed by boiling.
B. Vegetative cells grow best at cooking temperatures.
C. Extremely resistant to gastric acid.
D. Thick peptidoglycan renders cells resistant to heat, cold, and drying.
E. Surface proteins inhibit digestive hydrolases.

Questions 21-23: Gram-positive septicemia.

A 25-year-old man has high fever and elevated white count. Blood cultures are obtained. He is treated with a broad spectrum cephalosporin plus an aminoglycoside. The next day the laboratory reports that cultures contain catalase positive Gram-positive cocci in clusters. Identification of the organism and determination of antibiotic sensitivity are underway. Pending arrival of these results, the cephalosporin is replaced with vancomycin.

21. _____ What was the major rationale for this change?

A. Cephalosporins (as a class) have a narrower spectrum of action than vancomycin.
B. The lab results suggest staphylococci, many of which are resistant to cephalosporins.
C. Lysis of bacteria by cephalosporins may result in a massive inflammatory reaction.
D. Vancomycin is a bactericidal antibiotic whereas cephalosporins are only bacteriostatic.

The organism is identified as Staphylococcus aureus.
Antibiotic sensitivity testing results are as follows:

Penicillin G    R
Oxacillin (anti-staph penicillin)   R
Third-generation cephalosporin    R
Vancomycin   s
Erythromycin (macrolide)    R

Tobramycin (aminoglycoside)  R
Chloramphenicol   R
(metabolic inhibitor) s

22. ____ What is the most likely genetic/biochemical basis of this isolate’s resistance to oxacillin and the cephalosporin?

A. Small non-conjugative plasmid that encodes a penicillinase.
B. Conjugative plasmid, contains transposon that encodes a broad-spectrum β-lactamase.
C. DNA insert into chromosome, encodes a ‘penicillin-binding protein’ (= peptidoglycan cross-linking enzyme) resistant to β-lactams.
D. There is no D-ala-D-ala in the side chains of peptidoglycan.
E. No peptidoglycan is present in the cell wall.

23. ______ What essential process in Staphylococcus aureus is inhibited by Erythromycin?

A. Methylation of ribosomal RNA.
B. Production of DNA supercoils by Topoisomerase II
C. Cross-linking of peptidoglycan chains by transpeptidases.
D. Enzymatic activity of dihydrofolate reductase.
E. Peptide bond formation by bacterial ribosomes.

24. _____ A security guard at a New Jersey Court House comes to your office with a large lesion on his left arm. It is 10 cm in diameter and ulcerated. Surrounding tissue is red and markedly swollen. The ulcer is healing to produce a black scab. A swab of the ulcer contains large Gram-positive rods. After aerobic culture on sheep blood agar these produce large β-hemolytic colonies.
Which organism is most likely?

A. Bacillus anthracis
B. Bacillus cereus
C. Clostridium difficile
D. Clostridium perfingens
E. Clostridium tetani
F. Listeria monocytogenes

25._____ A 25-year-old medical student returns to New Jersey from Spring Break (which he spent fishing in North Carolina). Soon afterwards, he becomes ill. He reports a 5-day history of fever, malaise, and headache. He has three “Bull’s eye” lesions on his thighs. Lesions are 10-15 cm in diameter, oval with somewhat irregular outlines, with 3-4 cm red borders and 2-4 cm central red areas, separated by a belt of paler skin. Which diagnosis is most likely?

        A dermatophyte such as Trichophyton
        Lyme disease
        Relapsing fever
        Rocky Mountain spotted fever
        Staphylococcal scalded skin syndrome
        Primary syphilis
        Secondary syphilis

26. _____ A 12-year-old girl presents with fever, malaise, and a red, painful, swollen, right hand and prominent lymphadenopathy of the arm and axilla. The patient has a cat and reports that a few days ago the wretched ungrateful creature bit her on the right hand.
What pathogen is most strongly suggested by the history?

        An alpha-hemolytic Streptococcus
        Bartonella (=Rochalimea) henselae
        Brucella abortus
        Ehrlichia canis
        Francisella tularanesis
        Legionella pneumophila
        Pasteurella multocida
        Yersinia pestis
Questions 27-29: A young woman with an abscess.

A 23-year-old female college student went to the Student Health Service because of fever, myalgia, and a five-cm abscess: hot, red, swollen, and painful, with a ‘head’ of pus forming. This was lanced, drained, and disinfected. She was given antibiotics and acetaminophen (Tylenol). Late that day her roommate found her disoriented and took her to the Emergency Room. Her blood pressure was below normal and her pulse elevated. She showed no signs of meningitis but was lethargic and reported severe muscle aches. She was admitted. The next morning she was still hypotensive and showed signs of liver (jaundice) and kidney (poor urine output) dysfunction. Blood levels of liver and muscle enzymes were elevated. The preliminary diagnosis was Toxic Shock Syndrome.

27. _____ What process damaged liver, kidney, and muscle?

A. Bacteria infected these sites by gaining access to the bloodstream from the abscess.
B. A bacterial exotoxin acted on the central nervous system; organ damage was a secondary result.
C. Lipopolysaccharide released from dying bacteria caused monocytes to produce inflammatory cytokines.
D. A bacterial exotoxin stimulated production of toxic levels of inflammatory cytokines by T cells.
E. Bacteria bound to plasma membrane receptors of nerve, muscle, and kidney, and induced apoptosis.

28. _____ In a stained smear of pus from the abscess, what cell type would you expect to predominate?

A. Basophils
B. Eosinophils
C. Neutrophils
D. Lymphocytes
E. Monocytes

29. _____ What process was most largely responsible for the swelling around the abscess?

A. Dilation of small blood vessels
B. Increase in permeability of small blood vessels
C. Increased responsiveness of local sensory nerve endings
D. Proliferation of fibroblasts
E. Infiltration of tissue by inflammatory cells

30. _____ A medical student doing research accidentally stabs himself in the left wrist with a needle containing purified E. coli outer membranes. Two hours later he arrives at the Emergency Room with a red, swollen wrist and a fever of 40oC.

What process would you expect to have been most effective in producing his fever?
A. Binding of O-antigen by IgG antibodies
B. Binding of Toll-like Receptors by Lipid A
C. Binding of magnesium ions by core polysaccharide
D. Binding of peptidoglycan to macrophage ‘scavenger receptors’
E. Binding of porin proteins by siderophores.

31. _____ A patient presented with upper back pain. MRI imaging demonstrated inflammation of the T8-T9 vertebral bodies consistent with osteomyelitis. A bone marrow biopsy, when cultured, produced colonies of Gram-positive cocci. Colonies on Sheep blood agar were 3-4 mm in diameter, off-white and beta-hemolytic. Colonies on Brain-heart infusion (BHI) agar were 2-3 mm in diameter and golden yellow.
Catalase and coagulase tests were positive.
What is the most likely pathogen?

        Streptococcus pyogenes
        Streptococcus pneumoniae
        Streptococcus mitis (or a related ‘oral streptococcus’)
        Streptococcus agalactiae
        Staphylococcus epidermidis (or a related skin organism)
        Staphylococcus aureus

32. _____ Gram stain of material aspirated from a subcutaneous abscess contained huge numbers of neutrophils and clusters of branched, weakly-staining, Gram-positive filaments. The filaments were also weakly acid-fast. The organisms grew in aerobic culture.
What organism is most likely?

A. Actinomyces israelii
B. Nocardia asteroides
C. Sporothrix schenckii
D. An ‘environmental mycobacterium’
E. Corynebacterium diphtheriae
F. Bacillus cereus

33. _____ A food-borne outbreak of Salmonella enteritis takes place in central New Jersey and is traced to chicken parts sold by a supermarket chain. Isolates from all cases are of serotype O104:H6.
What bacterial structures correspond to “O” and “H” antigens?

     O-Antigen:                                                                                       H-antigen:
A. Carbohydrate chains of LPS                                                            Flagella
B. Polysaccharide                                                             capsule Common or adhesive pili
C. Porins of outer membrane                                                       Conjugation pili
D. Peptide chains of peptidoglycan                                    Non-pilus surface adhesins
E. Fibrillar proteins                                                              Teichoic acids of cell envelope

34. _____ A 24-year-old man comes to clinic with a severely swollen, red, and painful pharynx. A Gram-stained smear of exudate contains abundant neutrophils and slender Gram-negative rods. A sample is sent for culture and he is treated with a broad-spectrum antibiotic. The Gram-negative rod grows on Chocolate agar but not on standard Sheep blood agar when plates are cultured in room air.
Which of the following organisms is most likely?

A. Bordetella pertussis
B. Chlamydia pneumoniae
C. Haemophilus influenzae
D. Klebsiella pneumoniae
E. Legionella pneumophila

35. ____ Isolates of Neisseria meningitidis that cause septicemia/meningitis produce capsules. Isolates which produce less-serious disease usually lack capsules. Infection with non-encapsulated isolates results in protective immunity to them. The antibodies usually bind to bacterial surface proteins.
In the genesis of these protective antibodies, which of the following is essential?

A. B lymphocytes are stimulated in a “T-independent” fashion.
B. CD8+ lymphocytes proliferate.
C. Neutrophils process and present bacterial antigen to T cells.
D. Dendritic cells phagocytose and digest bacterial proteins.
E. Peptides from bacterial proteins appear on cell surfaces, bound to Class I MHC.

Questions 36-38: Déja vu all over again.

A 22-year-old college student with no significant previous illness comes to see you after a trip to Mexico. On his third day in Cancun he developed watery diarrhea which lasted two days. He had avoided salads and un-bottled water but drank iced soft drinks from a stand on the beach. He did not see a physician or take antibiotics. After returning he is well for about three weeks but again develops diarrhea, this time bloody. He reports pain on passage of stool, which is black and tarry. A stained smear of stool reveals many red and white cells, amoeboid forms which contain ingested erythrocytes, and cyst forms. Endoscopy reveals ulcerations of his colonic mucosa. He is started on an appropriate drug but two days later develops fever and septicemia, likely produced by spread of intestinal flora to the bloodstream from the ulcers in his colon. He is treated with a combination of broad-spectrum antibiotics and makes a full recovery. About a month later, he develops a second episode of bloody diarrhea. This time, no amoeboid forms can be seen in stool smears. Gram stain of stool now shows only ‘normal flora’.
Culture (on appropriate media) for bacterial pathogens is also negative.

Answer each question from the list of organisms below them.

36. _____ Which organism was most likely to have caused the patient’s original episode of watery diarrhea?

37. _____ Which organism is most likely to have caused the patient’s bloody diarrhea, three weeks after returning from Mexico?

38. _____ His course of broad-spectrum antibiotics should raise suspicion of what organism, as the cause of his third episode of diarrhea?

A. Balantidium coli
B. Campylobacter jejuni
C. Clostridium difficile
D. Cryptosporidium parvum
E. Entamobea histolytica
F. Escherichia coli
G. Giardia lamblia
H. Salmonella enterica
I. Shigella dysenteriae
J. Vibrio cholerae
Questions 39-40: A young woman with white lesions on her oral mucosa.

39. _____ A 28-year old woman comes to clinic because creamy white patches have developed on the lining of her oral cavity. A smear of a scraping from a lesion contains budding yeast and pseudohyphae. Culture grows out creamy white colonies of budding yeast. When yeast cells are suspended in plasma, buds elongate to form the beginnings of hyphae.

Which fungus is most likely in this setting?

A. Histoplasma capsulatum
B. Coccidiodes immitis.
C. Candida albicans
D. Cryptococcus neoformans
E. Paracoccidiodes braziliensis
F. Sporothrix schenckii
G. Pneumocystis carinii

40. _____ When the organism is examined under the microscope after culture on corn meal agar, it appears as sketched at right.

Which phrase best describes these forms?

A. Mixture of mold and yeast forms
B. Hyphae, arthroconidia, and microconidia
C. Hyphae, rhizoids, and sporangia
D. Hyphae, basidia, and chains of conidia
E. Hyphae, microconidia, and chlamydospores

Questions 41-43: A young boy with repeated bacterial infections.

A 8-month-old boy has a middle ear infection. Exudate contains Gram-positive cocci, many in pairs. Local lymph nodes and tonsils are barely visible; you would have expected both to be swollen. His medical history includes numerous similar infections caused by non-encapsulated Haemophilus influenzae, Streptococcus pneumoniae, and Moraxella catarrhalis, a Gram-negative coccus; pneumonia at 5 months, for which he was hospitalized; and a staphylococcal skin infection, which responded poorly to antibiotics, at 6 months. Blood cultures from the pneumonia grew Streptococcus pneumoniae. Family history reveals a male cousin who died at age 14 months from pneumonia.

41. _____ If the cause of his present illness is S. pneumoniae, what initial report would you expect from the lab?

A. G+ cocci, catalase-negative, α-hemolytic on sheep blood agar
B. G+ cocci, catalase-negative β-hemolytic on sheep blood agar
C. G+ cocci, catalase-negative, γ-hemolytic on sheep blood agar
D. G+ cocci, catalase-positive, α-hemolytic on sheep blood agar
E. G+ cocci, catalase-positive, β-hemolytic on sheep blood agar

42. _____ If the patient has an immunodeficiency syndrome, what aspect of the immune system is most likely to be defective?

A. IgM/IgG, complement, and neutrophils
B. IgE, mast cells, and basophils
C. Fucntion of CD4+ T cells
D. Proliferation and cytolysis by CD8+ cells
E. Natural killer cell function

43. _____ The patient remained healthy until he was five months old.

What is the most likely reason that he did not develop infections before the age of five months?

A. Babies of that age range are not exposed to bacteria.
B. Maternal IgG provided protection.
C. During the first few months of life the phagocytic capacity of neutrophils is high, but then declines.
D. Before5-6 months the intestine contains no ‘normal flora’.
E. Factors H and I are not produced during the first six months of life, so that Complement is more active then than later.

44. ____ A 20-year old college student with no previous history of significant illness comes to clinic because of ten days of low fever, dry cough, and malaise. Chest x-ray shows widespread diffuse inflammation of the lungs. She is given a prescription of an erythromycin-class drug, because a common bacterial cause of this type of illness in college students is resistant to all beta-lactam antibiotics. Which organism below best fits this description?

A. Staphylococcus aureus
B. Streptococcus pneumoniae
C. Coxiella burnetii
D. Mycoplasma pneumoniae
E. Chlamydia pneumonia

45. ____ Envelopes of Pseudomonas aeruginosa are much less permeable to antibiotics than envelopes of Escherichia coli. Which of the following is most likely to account for the difference?

A. Polar vs. peritrichous flagella
B. Thickness of peptidoglycan
C. Type of porin proteins in the outer membrane
D. Presence or absence of common pili
E. Type of teichoic acid present

46. ____ Isolates of Streptococcus pneumoniae which cause invasive disease are heavily-encapsulated. Isolates that lack capsules are much less virulent. How do capsules facilitate development of invasive disease?

A. Toxic to antibody-producing plasma cells.
B. Block transport of protein antigens to lymph nodes.
C. Prevent phagocytosis by neutrophils.
D. Inhibit cytokine production by Helper T cells.
E. Bind IgG and prevents activation of complement.

47. ____ An engineer recently returned from South America develops high fevers. Fever is nearly continuous but there is some indication that it peaks every other day. A blood smear prepared during a febrile interval is illustrated at right.

Which organism is the most likely cause of this illness?

A. Plasmodium falciparum
B. Plasmodium malariae
C. Plasmodium ovale
D. Plasmodium vixax
E. Babesia microti

48. ____ Salmonella enterica serotype typhi, unlike most other serotypes of Salmonella, can evade killing by macrophages. How does “S. typhi” evade killing?

A. Injects proteins into macrophage cytosol with a Type III secretion system
B. Secretes a protein exotoxin that kills macrophages
C. Once phagocytosed, secretes a phospholipase that destroys the membrane of the phagosome
D. Produces an extracellular adenyl cyclase that inhibits phagocytosis
E. Binds tightly to the outer face of the macrophage plasma membrane

49. ____ A 12-year-old boy develops a heart murmur. To help confirm or rule out Acute Rheumatic Fever, you should order a serological test for antibodies to a specific bacterial antigen.
In this test, which antigen should be uses?

A. Protein A of Staphylococcus aureus
B. M protein of Streptococcus pyogenes
C. α-toxin of Clostridium perfringens
D. Filamentous hemagglutinin (fha) of Bordetella pertussis
E. K1-antigen of Escherichia coli

50. ____ A tourist from France develops watery diarrhea produced by a local New Jersey strain of
enterotoxigenic E. coli (ETEC). How did the E. coli toxin produce diarrhea?

A. ADP-ribosylated a large G protein, stimulated mammalian adenyl cyclase
B. Cleaved ribosomal RNA, inhibited protein synthesis by 80S ribosomes
C. Protease, cleaved surface protein of membrane vesicles in cytoplasm of mammalian cells
D. Initiated polymerization of actin filaments adjacent to intracellular bacteria
E. Entered intestinal epithelial cells, synthesized cyclic GMP in cytosol

51. _____ A 23-year-old woman comes to your office because, for 3 days, she has experienced burning with urination, increased frequency of urination, and a continual feeling that she needs to urinate. She does not have vaginal discharge, fever, or flank pain. Rapid ‘dipstick’ urine tests are consistent with uncomplicated cystitis. Culture of urine on standard media produces a lactose-fermenting Gram negative rod.

In situations such as this, what is the most likely pathogen?

        Enterobacter faecium
        Escherichia coli
        Klebsiella pneumoniae
        Proteus vulgaris
        Pseudomonas aeruginosa
        Serratia marcescens
        Salmonella enterica
        Shigella boydii (or other Shigella)

52. _____ A burn patient develops infection of the burn wound. Culture of exudate on Brain-Heart Infusion agar produces numerous green-pigmented colonies of a Gram-negative rod. The organism grows well aerobically on standard media but shows no evidence of acid production on differential media such as MacConkey, Hektoen Enteric, or Triple-Sugar-Iron agars.

What is the most likely organism?

A. Bacteroides fragilis
B. Campylobacter fetus
C. Francisella tularensis
D. Pseudomonas aeruginosa
E. Escherichia coli

53. _____ A female patient comes to you because of vaginal itching and discharge. Microscopic examination of an unstained wet mount of discharge reveals squamous epithelial cells and the motile forms illustrated at right.

What organism is this?

A. A Microsporidian
B. Toxoplasma gondii
C. Cryptosporidium parvum
D. Giardia lamblia
E. Trichomonas vaginalis

54. _____ A patient develops fevers after returning to New Jersey from a safari vacation in East Africa. A blood smear, taken at the height of an episode of fever, is illustrated at right.
Which organism is most likely to be the cause of his fever?

A. Trypanosoma brucei brucei
B. Trypanosoma brucei rhodesiense
C. Trypanosoma brucei gambesiense
D. Trypanosoma cruzi
E. A dermotropic Leishmania
F. A Leishmania that produce visceral infections

55. _____ An HIV-positive patient develops chronic intractable diarrhea. Stool smear contains small acid-fast cyst forms. Which organism is the most likely cause of the patient’s diarrhea?

A. Toxoplasma gondii
B. Sarcocystis hominis
C. Cryptosporidium parvum
D. Isospora belli
E. Mycobacterium avium-intracellulare
F. Actinomyces israelii

56. ____ A 34-year-old man has fever and an extensive rash that extends to his palms and soles.
You order tests to confirm or rule out secondary Syphilis. If this is indeed secondary Syphilis, what results do you expect?

Test for anti-cardiolipin“non-treponemal” Abs      Test for antibodies specific forTreponema pallidum
A.             Positive                                                                                                          Negative
B.             Positive                                                                                                           Positive
C.             Negative                                                                                                         Negative
D.             Negative                                                                                                         Positive

57. ____ A young woman presents with cervicitis. A stained smear of material swabbed from the inflamed area contains cells with prominent peri-nuclear inclusions. Some inclusions are so large that they fill nearly the entire cytoplasm. What organism is the most likely cause of the cervicitis in this patient?

A. Chlamydia pneumoniae
B. Chlamydia trachomatis
C. Chlamydia psittaci
D. Mycoplasma pneumoniae
E. Mycoplasma hominis
F. Ureaplasma urealyticum

58. ____ You suspect that a patient with febrile illness and rash might have infection with a Rickettsia.
In a biopsy, in what cells would you would expect to find these organisms multiplying?

A. Pneumocytes (cells that line the alveoli of the lung)
B. Hepatocytes (liver cells)
C. B lymphocytes
D. Intestinal epithelial cells
E. Neurons
F. Endothelial cells

59. ____ An elderly man with chronic heart and lung disease develops Legionella pneumonia.
By what route was the infection most likely to have been acquired?

A. Respiratory route, from an infected person.
B. Inhalation of aerosolized water from a source containing protozoa.
C. Using snuff that contained spores.
D. Inhalation of spores from moldy hay.
E. Exposure to livestock or animal products.
F. Aerosolized cat or dog saliva.

60. ____ In coastal Massachusetts there have been several recent cases of Tularemia, acquired when a person operating a riding lawn mower ran over a dead or dying rabbit, and aerosolized micro-organisms from the animal. What property of Francisella tularensis makes possible its transmission in this bizarre fashion?

A. Short O-antigen chains on the outer membrane.
B. Production of an exotoxin with an extremely low lethal dose.
C. A lipid-rich cell envelope which makes bacteria resistant to dessication.
D. A very low infectious dose.
E. A capsule that has the same antigenic structure as human cell-surface oligosaccharides.

61. ____ A group of people eat dinner together at a local restaurant and about 1/3 of them become ill. Several, including two pregnant women, become septicemic and one of the women suffers a spontaneous abortion. A small, motile, aerobic, β-hemolytic, Gram-positive rod is recovered from blood and other cultures from many of those who became ill. It does not form spores.

Which organism listed below is most to be the pathogen?

A. Bacillus anthracis
B. Bacillus cereus
C. Leptospira interrogans
D. Borrelia recurrentis
E. Listeria monocytogenes

62. ____ A diabetic 78-year-old man develops a necrotic ulcer on his right foot. Infection spreads rapidly and there is gas in soft tissues. His foot is amputated at the ankle. Gram stain of exudate from the amputation site contains large Gram-positive rods. The bacteria grow well anaerobically, producing large β-hemolytic colonies on sheep blood agar, but do not grow aerobically.

What organism is most probable in this setting?

A. Bacillus anthracis
B. Bacillus cereus
C. Corynebacterium diphtheriae
D. Nocardia asteroides
E. Clostridium tetani
F. Clostridium perfringens

63. ____ A patient presents with a slowly-developing pneumonia. Several opacities are seen in a lung x-ray. On his second hospital day he develops high fever and hypotension. In a stained smear of leukocytes from the buffy coat of blood tiny intracellular yeast forms are seen. They are illustrated at right below. [When blood is centrifuged a thick layer of red erythrocytes collects at the bottom of the tube. Above this is a thin off-white layer that contains the white cells - the “buffy coat”. Above the buffy coat is the clear pale-yellow plasma.]

What organism is this most likely to be?

A. Coccidiodes immitis
B. Blastomyces dermatitidis
C. Histoplasma capsulatum
D. Cryptococcus neoformans
E. Pneumocystis carinii
F. Candida albicans

64. _____ A patient with a Gram-negative infection is treated with a cephalosporin, a β-lactam antibiotic.
In bacteria, what essential process is inhibited by β-lactams?

A. Synthesis of disaccharide precursors of peptidoglycan.
B. Transport of disaccharide precursors through the plasma membrane.
C. Polymerization of peptidoglycan disaccharides into long chains.
D. Conversion of L-alanine to D-alanine.
E. Polymerization of the peptide side chains of peptidoglycan.
F. Cross-linking of the peptide side chains of peptidoglycan.

65. ____ For an isolate of E. coli obtained from a blood culture, the lab reports antibiotic Minimal Inhibitory Concentrations (MIC) as follows:

Drug                                                                 MIC (μg/ml)
Ampicillin (penicilin)                                              3
Ceftazidime (cephalosporin)                              0.03
Tetracycline                                                           128

Which conclusion best follows from these data?

A. On this isolate, ampicillin will have only a bacteriostatic effect.
B. The isolate is resistant to cephalosporins because it produces a TEM-type β-lactamase
C. Of these three drugs, tetracycline will be the most effective against this isolate.
D. Of these three drugs, ceftazidime will kill this isolate at the lowest concentration.
E. When used in combination, ampicillin and tetracycline will have synergistic effects.

Q: 71-77

Match each of the following agents with the corresponding features (choices may be used once, more than once or not at all):

A.  Herpes simplex 1

B.  Herpes simplex 2

C.  Varicella-zoster virus

D.  Cytomegalovirus

E.  Epstein-Barr virus (EBV)

F.  Human Herpesvirus 8 (HHV8)

71. _____ A child born deaf.(D)

72. _____ An AIDS patient presents with Kaposi’s Sarcoma (F)

73. _____ A 72-year-old male presents with a red blistery rash along the distribution of the 8th and 9th thoracic spinal nerve.(C)

74. _____ A sexually active 24-year-old patient presents with a blistery rash on the genitals.(B)

75. _____ A patient presents with severe encephalitis.(A & B)

76. _____ A 6-year-old child presents with severe rash and blistering on the cheeks, lips and in the mouth.(A)

77. _____ An adolescent patient presents with fatigue, fever, sore throat, enlargement of lymph nodes and an enlarged spleen.(E)

Q: 78-80

Match each of the following agents with the corresponding features (choices may be used once, more than once or not at all):

A.  Clostridium tetani (A)

B.  Clostridium botulinum (B)

C.   Clostridium perfringens (C)


78. _____ Spastic paralysis                                 

79. _____ Flaccid paralysis                                             

80. _____ Gas-gangrene     

1 comment: