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USMLE Step 1 Microbiology: High-Yield Bacteria, Viruses and Fungi — quiz
15 multiple-choice questions covering Staphylococcus aureus, Streptococcus pyogenes, Clostridium botulinum vs tetani, Corynebacterium diphtheriae, Bacillus anthracis. One answer each; you get a score at the end and the explanation for every question. No account needed.
Updated 2026-11-04. Keys 1–4 pick an answer, Enter moves on. Redo it any time, in order or shuffled.
The questions
Answers and explanations are shown after you finish.
- Staphylococcus aureus
A patient develops fever, hypotension, and a diffuse sunburn-like rash after prolonged tampon use. Blood cultures grow coagulase-positive, catalase-positive cocci in clusters. Which toxin is most directly responsible for this presentation?
- Shiga-like toxin
- Exfoliatin
- TSST-1
- Protein A
- Streptococcus pyogenes
A throat culture from a child with pharyngitis grows beta-hemolytic, catalase-negative cocci that are sensitive to bacitracin. Which organism is this?
- Streptococcus pyogenes
- Streptococcus agalactiae
- Enterococcus faecalis
- Streptococcus pneumoniae
- Clostridium botulinum vs tetani
A patient develops descending flaccid paralysis after eating improperly canned vegetables. Compared to tetanus toxin, the toxin responsible here acts by which mechanism?
- Blocking acetylcholine release at the neuromuscular junction
- Increasing cAMP via Gs activation
- Inhibiting elongation factor-2
- Blocking glycine/GABA release in the spinal cord
- Corynebacterium diphtheriae
A child with a gray pharyngeal pseudomembrane and myocarditis is diagnosed with a toxin-mediated infection. The toxin responsible causes cell death by which mechanism?
- Blocking glycine release in the spinal cord
- ADP-ribosylating elongation factor-2, halting protein synthesis
- Increasing cAMP via permanent Gs activation
- Inhibiting the 60S ribosomal subunit directly
- Bacillus anthracis
A wool-mill worker develops flu-like symptoms that progress rapidly to severe respiratory distress with a widened mediastinum on chest x-ray. Which organism is the likely cause?
- Clostridium perfringens
- Mycobacterium tuberculosis
- Bacillus anthracis
- Bacillus cereus
- Neisseria gonorrhoeae
A sexually active patient has purulent urethral discharge. Gram-negative diplococci grow on Thayer-Martin agar but fail to ferment maltose. Compared with Neisseria meningitidis, this organism is distinguished by which feature?
- It is gram-positive
- It causes Waterhouse-Friderichsen syndrome
- It lacks a polysaccharide capsule
- It ferments maltose
- Salmonella vs Shigella
Compared with Salmonella, an infection with Shigella is characterized by which combination of features?
- H2S production and motility
- Lower infectious dose and non-motility
- Higher infectious dose and motility
- Frequent bacteremia
- Klebsiella pneumoniae
An alcoholic patient develops aspiration pneumonia with thick, blood-tinged ('currant jelly') sputum. Gram-negative encapsulated lactose-fermenting rods are cultured. Which virulence factor best explains the viscous sputum?
- Pili
- Polysaccharide capsule
- Exotoxin A
- Lipid A endotoxin
- Vibrio cholerae
A patient with profuse rice-water diarrhea after drinking contaminated water is found to have a toxin that causes massive fluid secretion. What is the toxin's mechanism?
- ADP-ribosylating elongation factor-2
- Increasing cGMP levels
- Permanently activating the Gs protein, raising cAMP
- Inhibiting the inhibitory Gi protein
- Bordetella pertussis
An unvaccinated child with paroxysmal coughing and a notable lymphocytosis is diagnosed with pertussis. The pertussis toxin impairs immune cell chemotaxis by which mechanism?
- Blocking acetylcholine release
- Disabling the inhibitory Gi protein, raising cAMP
- ADP-ribosylating elongation factor-2
- Activating the Gs protein
- Treponema pallidum
A patient has a positive nontreponemal RPR test for syphilis. Which test should be used to confirm this diagnosis?
- Urease breath test
- VDRL
- FTA-ABS
- Monospot test
- Chlamydia trachomatis
A patient being treated for gonorrhea with ceftriaxone is also given azithromycin. What is the main reason for adding azithromycin?
- Chlamydia trachomatis has peptidoglycan requiring beta-lactams
- To prevent progression to neurosyphilis
- To reduce development of antibiotic resistance
- To cover possible co-infection with Chlamydia trachomatis
- Epstein-Barr virus
A college student with fever, pharyngitis, and posterior cervical lymphadenopathy is mistakenly treated with amoxicillin for strep throat and develops a diffuse rash. What is the most likely underlying diagnosis?
- Scarlet fever
- Cytomegalovirus infection
- Epstein-Barr virus mononucleosis
- Streptococcal pharyngitis
- HIV
In a patient newly diagnosed with HIV, which viral enzyme is responsible for converting the ssRNA genome into DNA before it integrates into the host chromatin?
- gp120 binding to CD4
- Integrase
- Protease
- Reverse transcriptase
- Aspergillus fumigatus
A patient with a history of cavitary tuberculosis develops hemoptysis. Imaging reveals a mobile mass within a pre-existing lung cavity, and microscopy shows septate hyphae branching at acute angles. What is the diagnosis?
- Histoplasmosis
- Cryptococcosis
- Mucormycosis
- Aspergilloma (Aspergillus fumigatus)
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