USMLE Step 1 Microbiology Practice Questions

A previously healthy college student presents with fever, headache, nuchal rigidity, and a petechial rash. Gram stain of cerebrospinal fluid (CSF) reveals Gram-negative diplococci. Which organism is the most likely cause?

• (A) Neisseria meningitidis
• (B) Streptococcus pneumoniae
• (C) Haemophilus influenzae type b
• (D) Listeria monocytogenes

Which bacterium is characterized by Gram-positive cocci arranged in clusters, catalase-positive, coagulase-positive, and is a common cause of skin infections, abscesses, pneumonia, and sepsis?

• (A) Staphylococcus aureus
• (B) Staphylococcus epidermidis
• (C) Streptococcus pyogenes (Group A Strep)
• (D) Enterococcus faecalis

Which virus is responsible for causing infectious mononucleosis, characterized by fever, pharyngitis, lymphadenopathy, splenomegaly, and atypical lymphocytes (Downey cells) on peripheral blood smear?

• (A) Epstein-Barr virus (EBV)
• (B) Cytomegalovirus (CMV)
• (C) Human Immunodeficiency Virus (HIV)
• (D) Influenza virus

A patient develops profuse, watery diarrhea described as 'rice-water stool' after consuming contaminated seafood during travel to Southeast Asia. Which toxin-producing bacterium is the most likely causative agent?

• (A) Vibrio cholerae
• (B) Clostridium difficile
• (C) Enterotoxigenic E. coli (ETEC)
• (D) Shigella dysenteriae

Which fungal organism, often found in soil enriched with bird or bat droppings, typically causes pulmonary infection that can disseminate, particularly in immunocompromised individuals (e.g., HIV/AIDS), and is identified as an encapsulated yeast on India ink stain of CSF?

• (A) Cryptococcus neoformans
• (B) Histoplasma capsulatum
• (C) Coccidioides immitis
• (D) Blastomyces dermatitidis

Which parasite, transmitted by the bite of an infected female Anopheles mosquito, is responsible for causing malaria, characterized by cyclical fevers, chills, sweats, and hemolysis?

• (A) Plasmodium species (e.g., P. falciparum, P. vivax)
• (B) Trypanosoma cruzi
• (C) Leishmania donovani
• (D) Toxoplasma gondii

What is the mechanism of action of the botulinum toxin produced by Clostridium botulinum?

• (A) Inhibits acetylcholine (ACh) release at the neuromuscular junction, causing flaccid paralysis
• (B) Blocks glycine and GABA release in the spinal cord, causing spastic paralysis
• (C) ADP-ribosylates Gs protein, increasing cAMP and causing watery diarrhea
• (D) Superantigen causing massive T-cell activation

Which bacterium is a Gram-negative rod, oxidase-positive, grows on buffered charcoal yeast extract (BCYE) agar, and is associated with atypical pneumonia (Legionnaires' disease), often linked to contaminated water sources (e.g., air conditioning systems)?

• (A) Legionella pneumophila
• (B) Pseudomonas aeruginosa
• (C) Klebsiella pneumoniae
• (D) Mycoplasma pneumoniae

Which virus is the most common cause of the common cold, characterized by rhinitis, nasal congestion, and sore throat?

• (A) Rhinovirus
• (B) Influenza virus
• (C) Respiratory Syncytial Virus (RSV)
• (D) Adenovirus

A patient develops bloody diarrhea, abdominal cramps, and fever after consuming undercooked ground beef. Stool culture grows Escherichia coli O157:H7, which produces a Shiga-like toxin. This pathogen can lead to which serious complication, especially in children?

• (A) Hemolytic Uremic Syndrome (HUS)
• (B) Guillain-Barré syndrome
• (C) Toxic shock syndrome
• (D) Reactive arthritis

Which obligate intracellular bacterium, lacking a cell wall and not visible on Gram stain, is a common cause of atypical 'walking' pneumonia, particularly in young adults and military recruits?

• (A) Mycoplasma pneumoniae
• (B) Chlamydia pneumoniae
• (C) Legionella pneumophila
• (D) Coxiella burnetii

Which organism is characterized as a Gram-positive, spore-forming anaerobic rod and produces toxins that cause tetanus (spastic paralysis) or botulism (flaccid paralysis)?

• (A) Clostridium species (C. tetani, C. botulinum)
• (B) Bacillus anthracis
• (C) Listeria monocytogenes
• (D) Corynebacterium diphtheriae

Which virus uses reverse transcriptase to convert its RNA genome into DNA, which is then integrated into the host cell genome?

• (A) Human Immunodeficiency Virus (HIV)
• (B) Hepatitis C Virus (HCV)
• (C) Influenza virus
• (D) Herpes Simplex Virus (HSV)

A Gram stain of sputum from a patient with pneumonia shows Gram-positive lancet-shaped diplococci. Which bacterium is the most likely pathogen?

• (A) Streptococcus pneumoniae
• (B) Staphylococcus aureus
• (C) Klebsiella pneumoniae
• (D) Haemophilus influenzae

Which dimorphic fungus, endemic to the Ohio and Mississippi River valleys, exists as a mold in the environment (soil with bird/bat guano) and as a small yeast inside macrophages in the body, often causing pulmonary or disseminated disease?

• (A) Histoplasma capsulatum
• (B) Coccidioides immitis
• (C) Blastomyces dermatitidis
• (D) Cryptococcus neoformans

Which spirochete bacterium, transmitted by the bite of Ixodes ticks, is the causative agent of Lyme disease?

• (A) Borrelia burgdorferi
• (B) Treponema pallidum
• (C) Leptospira interrogans
• (D) Rickettsia rickettsii

What is the mechanism of action of diphtheria toxin produced by Corynebacterium diphtheriae?

• (A) Inhibits protein synthesis by ADP-ribosylating elongation factor 2 (EF-2)
• (B) Activates adenylate cyclase via ADP-ribosylation of Gs protein
• (C) Cleaves SNARE proteins, blocking neurotransmitter release
• (D) Forms pores in cell membranes

Which DNA virus family includes Herpes Simplex Virus (HSV-1, HSV-2), Varicella-Zoster Virus (VZV), Epstein-Barr Virus (EBV), Cytomegalovirus (CMV), and Human Herpesvirus 8 (HHV-8)?

• (A) Herpesviridae
• (B) Poxviridae
• (C) Adenoviridae
• (D) Papillomaviridae

A patient presents with fever, severe muscle aches (myalgias), headache, and a rash that starts on the wrists and ankles and spreads centrally. He reports being bitten by a tick while hiking in North Carolina. Which organism is the most likely cause?

• (A) Rickettsia rickettsii
• (B) Borrelia burgdorferi
• (C) Ehrlichia chaffeensis
• (D) Francisella tularensis

Which protozoan parasite, typically acquired by ingesting cysts in contaminated food or water (often associated with hikers drinking untreated water), causes giardiasis, characterized by foul-smelling, fatty stools (steatorrhea), bloating, and flatulence?

• (A) Giardia lamblia (also G. duodenalis or G. intestinalis)
• (B) Entamoeba histolytica
• (C) Cryptosporidium parvum
• (D) Trichomonas vaginalis

Which bacterium is a common cause of urinary tract infections (UTIs), particularly in young, sexually active women, and is characterized by Gram-negative rods, lactose fermentation (pink colonies on MacConkey agar), and indole positivity?

• (A) Escherichia coli
• (B) Klebsiella pneumoniae
• (C) Proteus mirabilis
• (D) Pseudomonas aeruginosa

Which virus is the leading cause of gastroenteritis (vomiting and diarrhea) in infants and young children worldwide, for which effective vaccines are now available?

• (A) Rotavirus
• (B) Norovirus
• (C) Adenovirus
• (D) Astrovirus

Which Gram-positive rod exhibits characteristic 'tumbling' motility at room temperature, can grow at refrigeration temperatures, is facultative intracellular, and can cause meningitis, sepsis, or gastroenteritis, particularly in pregnant women, newborns, and immunocompromised individuals?

• (A) Listeria monocytogenes
• (B) Corynebacterium diphtheriae
• (C) Bacillus cereus
• (D) Clostridium perfringens

Which helminth (worm) infection is typically acquired by ingesting eggs found in contaminated soil, leading to larvae hatching in the intestine, migrating through the lungs, being swallowed, and maturing into adult worms in the small intestine, potentially causing intestinal obstruction or biliary disease?

• (A) Ascaris lumbricoides (roundworm)
• (B) Necator americanus / Ancylostoma duodenale (hookworm)
• (C) Trichuris trichiura (whipworm)
• (D) Enterobius vermicularis (pinworm)

What is the primary virulence factor of Streptococcus pneumoniae that inhibits phagocytosis?

• (A) Polysaccharide capsule
• (B) M protein
• (C) Protein A
• (D) IgA protease

Which virus is the causative agent of chickenpox (varicella) and shingles (herpes zoster)?

• (A) Varicella-Zoster Virus (VZV / HHV-3)
• (B) Herpes Simplex Virus 1 (HSV-1)
• (C) Measles virus (Rubeola)
• (D) Rubella virus

Which bacterium, characterized by Gram-negative rods, oxidase-negative, non-lactose fermenting, H2S production, and flagellar motility, is a common cause of gastroenteritis (often from poultry/eggs) and can also cause typhoid fever?

• (A) Salmonella species (non-typhoidal Salmonella, S. Typhi)
• (B) Shigella species
• (C) Escherichia coli
• (D) Campylobacter jejuni

Which fungal infection, caused by Malassezia species (part of normal skin flora), presents as hypopigmented or hyperpigmented macules, usually on the trunk, and shows a 'spaghetti and meatballs' appearance (hyphae and yeast cells) on KOH preparation of skin scrapings?

• (A) Tinea versicolor (Pityriasis versicolor)
• (B) Tinea corporis (ringworm)
• (C) Candidiasis
• (D) Dermatophytosis

Which obligate intracellular bacteria are transmitted by arthropod vectors (ticks, mites, lice, fleas) and cause diseases like Rocky Mountain Spotted Fever, typhus, and ehrlichiosis?

• (A) Rickettsia and related genera (e.g., Ehrlichia, Anaplasma)
• (B) Borrelia
• (C) Chlamydia
• (D) Mycoplasma

What is the primary target cell for Human Immunodeficiency Virus (HIV)?

• (A) CD4+ T lymphocytes (Helper T cells)
• (B) CD8+ T lymphocytes (Cytotoxic T cells)
• (C) B lymphocytes
• (D) Neutrophils

Which bacterium causes peptic ulcer disease and gastritis, and is characterized by being a curved Gram-negative rod, microaerophilic, urease-positive, catalase-positive, and oxidase-positive?

• (A) Helicobacter pylori
• (B) Campylobacter jejuni
• (C) Vibrio cholerae
• (D) Pseudomonas aeruginosa

Which parasite, transmitted via the fecal-oral route through ingestion of oocysts, causes cryptosporidiosis, typically a self-limited watery diarrhea in immunocompetent hosts but potentially severe and chronic diarrhea in AIDS patients?

• (A) Cryptosporidium parvum / hominis
• (B) Giardia lamblia
• (C) Entamoeba histolytica
• (D) Cyclospora cayetanensis

Which group of antibiotics inhibits bacterial protein synthesis by binding to the 30S ribosomal subunit?

• (A) Aminoglycosides (e.g., gentamicin) and Tetracyclines (e.g., doxycycline)
• (B) Macrolides (e.g., azithromycin) and Clindamycin
• (C) Fluoroquinolones (e.g., ciprofloxacin)
• (D) Penicillins and Cephalosporins

Prions are infectious agents composed solely of:

• (A) Abnormally folded proteins (PrPSc)
• (B) Single-stranded RNA
• (C) Double-stranded DNA
• (D) Lipopolysaccharide

Which virus family includes non-enveloped, double-stranded DNA viruses that can cause pharyngitis, conjunctivitis ('pink eye'), gastroenteritis, pneumonia, and are used as vectors in gene therapy?

• (A) Adenoviridae
• (B) Picornaviridae
• (C) Paramyxoviridae
• (D) Orthomyxoviridae

Which anaerobic, Gram-positive bacterium, part of the normal gut flora, can overgrow after antibiotic therapy and produce toxins (Toxin A - enterotoxin, Toxin B - cytotoxin) that cause pseudomembranous colitis?

• (A) Clostridioides difficile (formerly Clostridium difficile)
• (B) Bacteroides fragilis
• (C) Peptostreptococcus
• (D) Actinomyces israelii

What is the characteristic feature used to classify Streptococcus species based on their hemolytic activity on blood agar?

• (A) Type of hemolysis (alpha, beta, gamma)
• (B) Coagulase production
• (C) Catalase production
• (D) Optochin sensitivity

Which opportunistic fungus, commonly found in the environment (e.g., decaying vegetation, air), typically exists as septate hyphae branching at acute angles, and can cause invasive pulmonary disease, allergic bronchopulmonary aspergillosis (ABPA), or aspergillomas in pre-existing lung cavities?

• (A) Aspergillus fumigatus (and other Aspergillus species)
• (B) Candida albicans
• (C) Mucor species (Mucormycosis)
• (D) Pneumocystis jirovecii

Syphilis is caused by which bacterium?

• (A) Treponema pallidum
• (B) Neisseria gonorrhoeae
• (C) Chlamydia trachomatis
• (D) Haemophilus ducreyi

Which virulence factor, produced by Staphylococcus aureus, binds to the Fc region of IgG antibodies, inhibiting opsonization and phagocytosis?

• (A) Protein A
• (B) Coagulase
• (C) Hemolysins
• (D) Hyaluronidase