USMLE Step 1 · Pathology and Microbiology

Microbiology and Immunology

34 practice questions, each with the answer and an explanation.

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Random questions from this topic, one at a time. You see the answer and the explanation straight after each one.

All 34 questions

  1. A 30-year-old woman has dysuria. Urinalysis shows nitrites and leukocyte esterase. Which organism is most common?
  2. A patient has an abscess with gram-positive cocci in clusters that are catalase and coagulase positive. Which organism is it?
  3. A man has a painless genital ulcer with a clean base. Which organism is the cause?
  4. Which immunoglobulin is the first made in a primary response?
  5. Which immunoglobulin crosses the placenta?
  6. A hiker has a bull's-eye rash and then facial palsy. Which organism is the cause?
  7. A patient with Goodpasture syndrome has antibodies against basement membrane collagen. Which hypersensitivity type is this?
  8. A traveler has cyclic fevers, anemia and ring forms in red blood cells. Which organism is responsible?
  9. A patient has a positive HBsAg and positive anti-HBc IgM. What is the interpretation?
  10. A 6-year-old has bloody diarrhea after eating undercooked beef and develops hemolytic anemia and renal failure. Which toxin is responsible?
  11. A patient has a CD4 count of 80 per microliter and headache. India ink stain of CSF shows encapsulated yeast. Which organism is responsible?
  12. A sexually active 20-year-old woman has dysuria, discharge and pelvic pain. Gram stain shows intracellular gram-negative diplococci. Which organism is responsible?
  13. A 20-year-old has fever, sore throat and posterior cervical lymphadenopathy. Peripheral smear shows atypical lymphocytes. Which test is positive?
  14. A 6-month-old infant has a barking cough and stridor, and chest X-ray shows a steeple sign. Which virus is the cause?
  15. A 25-year-old man has fever, neck stiffness and petechiae. CSF shows gram-negative diplococci. Which organism is most likely?
  16. A patient has rice-water stools and severe dehydration. What is the mechanism of the toxin?
  17. A patient with AIDS has dyspnea and bilateral ground-glass infiltrates. Silver stain shows cysts. Which organism is responsible?
  18. A child has a gray pseudomembrane in the throat and a bull neck. Which organism causes this?
  19. A 10-year-old has a sore throat and a sandpaper rash. Which toxin causes the rash?
  20. A patient develops hives and wheezing minutes after a bee sting. Which hypersensitivity type is this?
  21. A child has eczema, thrombocytopenia and recurrent infections. What is the diagnosis?
  22. A boy has recurrent staphylococcal abscesses and a negative nitroblue tetrazolium test. What is the defect?
  23. Which cell surface marker identifies helper T cells?
  24. A neonate has a high fever and meningitis. CSF culture grows gram-positive rods with tumbling motility. Which organism is responsible?
  25. Which hypersensitivity type is responsible for contact dermatitis from poison ivy?
  26. A 3-year-old has high fever for 3 days followed by a rash after the fever resolves. Which virus is the cause?
  27. A child has recurrent bacterial infections, absent tonsils and no B cells. Which gene is mutated?
  28. A hotel guest develops pneumonia with diarrhea and hyponatremia. Which organism requires charcoal yeast agar?
  29. A newborn has hypocalcemic tetany, a heart defect and absent thymic shadow. Which syndrome is most likely?
  30. A 25-year-old man has a rash on his wrists and ankles that spreads centrally after a camping trip, with fever. Which organism is responsible?
  31. A child has fever, a slapped-cheek rash and aplastic crisis in sickle cell disease. Which virus is the cause?
  32. A patient develops watery diarrhea after antibiotic use. Toxin assay is positive. Which organism is responsible?
  33. A 65-year-old alcoholic has fever, cough and currant jelly sputum. Which organism is most likely?
  34. A 20-year-old college student has a dry cough, malaise and patchy infiltrates. Cold agglutinins are positive. Which organism is most likely?

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