USMLE Step 1 · Pathology and Microbiology

Microbiology and Infectious Disease

22 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 22 questions

  1. A newborn has conjunctivitis 3 days after delivery. The Gram stain shows gram-negative intracellular diplococci. Which organism is responsible?
  2. Which organism is the most common cause of community-acquired pneumonia in adults?
  3. A 22-year-old man has a sore throat, fever, posterior cervical lymphadenopathy and splenomegaly. Atypical lymphocytes are seen on the smear. Which test confirms the cause?
  4. A farmer has a painless black eschar on the arm with surrounding edema. The Gram stain of the lesion shows large gram-positive rods. Which organism is responsible?
  5. A 25-year-old woman has dysuria and frequency. Urine culture grows a lactose-fermenting gram-negative rod that forms pink colonies on MacConkey agar. Which organism is the most likely cause?
  6. A man with a urinary catheter develops a urinary infection with alkaline urine and struvite stones. Which organism produces urease and causes this?
  7. A traveler has watery diarrhea that looks like rice water, with rapid dehydration. Which toxin mechanism explains the diarrhea?
  8. A child has a gray pseudomembrane over the tonsils and a bull neck. Which toxin action causes the disease?
  9. A 30-year-old man has a painless genital ulcer with a clean base and firm edges, and a dark-field examination shows spirochetes. What is the diagnosis?
  10. Which bacterium causes erythema migrans after a tick bite in the northeastern United States?
  11. A patient with HIV and a CD4 count of 50 cells/mm3 has a headache, and cerebrospinal fluid shows encapsulated yeast with India ink staining. Which organism is responsible?
  12. A patient with HIV and a CD4 count of 80 cells/mm3 has dyspnea and a dry cough. The chest X-ray shows bilateral ground-glass opacities. Which prophylaxis would have prevented this?
  13. A 20-year-old college student has a fever, headache, neck stiffness and a petechial rash. Gram stain of the cerebrospinal fluid shows gram-negative diplococci. Which prophylaxis is given to close cont…
  14. A child has intense itching around the anus at night, and a piece of tape applied to the skin shows eggs. Which drug is the treatment of choice?
  15. A traveler returns from sub-Saharan Africa with cyclical fevers every 48 hours and a ring form inside red cells on a blood smear. Which Plasmodium species causes tertian fever with dormant liver stage…
  16. Which virus is the most common cause of bronchiolitis in infants?
  17. Which hepatitis B serologic pattern indicates a patient who was successfully vaccinated?
  18. A man develops diarrhea and fever, and a stool culture shows a gram-negative rod that does not ferment lactose and does not produce hydrogen sulfide. Which organism is the most likely?
  19. A patient has a fever and a new murmur after dental extraction. Blood cultures grow alpha-hemolytic, optochin-resistant cocci that are catalase-negative. Which organism is the most likely cause of thi…
  20. A 25-year-old woman has a vaginal discharge with a fishy odor, and clue cells are seen on microscopy. What is the best treatment?
  21. A child has a honey-colored crusted skin lesion around the nose. Which organisms are the most common causes?
  22. A patient with cystic fibrosis has a chronic lung infection with a gram-negative rod that produces a blue-green pigment and a grape-like odor. Which organism is responsible?

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