USMLE Step 1 Microbiology Exam Prep | Practice Questions &
Answers with Detailed Rationales Instant Download
EXAM COVERAGE SUMMARY
Bacterial Structure & Staining: Cell wall components (pili, capsules, periplasm), Gram-
positive vs Gram-negative, acid-fast staining, special stains (Giemsa, PAS, Ziehl-Neelsen,
India ink, Silver)
Bacterial Classification & Growth: Aerobic vs anaerobic, obligate intracellular vs
facultative intracellular, encapsulated bacteria, urease-positive, catalase-positive organisms
Bacterial Virulence Factors: Protein A, M protein, IgA protease, Type III secretion system,
exotoxins vs endotoxins (mechanisms, effects, genetics)
Bacterial Toxins: ADP-ribosylating toxins (cholera, pertussis, diphtheria, Pseudomonas
exotoxin A), Shiga toxin, SNARE-cleaving toxins (tetanus, botulinum), superantigens (TSST-
1, streptococcal exotoxin), membrane-disrupting toxins
Specific Bacterial Pathogens: Gram-positive (Staphylococcus, Streptococcus, Bacillus,
Clostridium, Listeria, Corynebacterium, Actinomyces, Nocardia, Mycobacterium), Gram-
negative (Neisseria, Haemophilus, Bordetella, Legionella, Pseudomonas, Enterobacteriaceae,
Vibrio, Campylobacter, Helicobacter, Yersinia, Brucella, Francisella), Spirochetes
(Treponema, Borrelia, Leptospira), Chlamydia, Rickettsia, Mycoplasma
Fungal Pathogens: Systemic mycoses (Histoplasma, Blastomyces, Coccidioides,
Paracoccidioides), Opportunistic fungi (Candida, Aspergillus, Cryptococcus,
Mucor/Rhizopus, Pneumocystis, Sporothrix), Dermatophytes
Parasitic Infections: Protozoa (Giardia, Entamoeba, Cryptosporidium, Toxoplasma,
Plasmodium, Babesia, Trypanosoma, Leishmania, Trichomonas, Naegleria), Helminths
(nematodes, cestodes, trematodes), Ectoparasites
Virology Fundamentals: Viral structure, replication, genetics (recombination, reassortment,
complementation, phenotypic mixing), DNA vs RNA viruses, positive vs negative sense,
segmented vs non-segmented
Specific DNA Viruses: Herpesviruses (HSV-1, HSV-2, VZV, EBV, CMV, HHV-6/7, HHV-
8), Papillomavirus, Polyomavirus (JC, BK), Parvovirus B19, Adenovirus, Poxvirus,
Hepadnavirus (HBV)
Specific RNA Viruses: Picornaviruses, Flaviviruses, Togaviruses, Retroviruses (HIV),
Orthomyxoviruses (Influenza), Paramyxoviruses, Rhabdoviruses (Rabies), Filoviruses
(Ebola), Coronaviruses, Reoviruses (Rotavirus), Hepatitis viruses (HAV, HCV, HDV, HEV)
HIV/AIDS: Pathogenesis, CD4+ count correlation with opportunistic infections, diagnosis,
associated malignancies
Prion Diseases: Pathogenesis, clinical manifestations
Diagnostic Methods: Culture media, staining techniques, serology
Treatment Principles: Antimicrobial agents, resistance mechanisms
Epidemiology & Transmission: Nosocomial infections, food-borne illnesses, vector-borne
diseases
, Host Defense & Immunology: Immune response to infections, vaccines, hypersensitivity
reactions
250 MICROBIOLOGY QUESTIONS
Question 1
A 45-year-old farmer presents with a painless ulcer on his forearm that has developed a black eschar.
He reports handling animal hides. Which virulence factor is most responsible for the characteristic
edematous appearance of this lesion?
A) Tetanospasmin
B) Edema toxin
C) Alpha toxin
D) Shiga toxin
E) Cholera toxin
Answer: B) Bacillus anthracis produces edema toxin that mimics adenylate cyclase, increasing
cAMP and causing localized edema. The black eschar is characteristic of cutaneous anthrax, and the
edema toxin is responsible for the surrounding edematous borders. Tetanospasmin causes spastic
paralysis, alpha toxin causes gas gangrene, Shiga toxin causes dysentery, and cholera toxin causes
watery diarrhea.
Question 2
A 28-year-old woman develops fever, hypotension, and a diffuse erythematous rash during her
menstrual period. She is found to have tampon use. Which bacterial superantigen is most likely
responsible?
A) Streptolysin O
B) Toxic shock syndrome toxin-1
C) Alpha toxin
D) Botulinum toxin
E) Diphtheria toxin
Answer: B) Staphylococcus aureus produces TSST-1, a superantigen that cross-links MHC class II
molecules with T-cell receptors outside the antigen-binding site, causing massive cytokine release (IL-
1, IL-2, IFN-γ, TNF-α) leading to fever, rash, and shock. This is classic toxic shock syndrome
,associated with tampon use. Streptolysin O is from S. pyogenes, alpha toxin from C. perfringens,
botulinum toxin causes flaccid paralysis, and diphtheria toxin inhibits EF-2.
Question 3
A 6-month-old infant presents with a "floppy" appearance, poor feeding, and constipation. The mother
reports feeding the infant honey. Which mechanism best describes the action of the causative toxin?
A) Blocks release of inhibitory neurotransmitters
B) Blocks release of acetylcholine at neuromuscular junctions
C) Inhibits elongation factor-2
D) Activates adenylate cyclase
E) Cleaves IgA
Answer: B) Clostridium botulinum produces botulinum toxin, a protease that cleaves SNARE
proteins, preventing vesicle fusion and release of acetylcholine at neuromuscular junctions. This
causes flaccid paralysis, manifesting as the "floppy baby" syndrome in infants who ingest spores from
honey. Tetanospasmin blocks inhibitory neurotransmitters (GABA/glycine), diphtheria toxin inhibits
EF-2, cholera toxin activates adenylate cyclase, and IgA protease cleaves IgA antibodies.
Question 4
A 32-year-old man who recently returned from a camping trip develops explosive, watery diarrhea
described as "rice water" stools. Which bacterial toxin mechanism is responsible?
A) Inactivation of 60S ribosome
B) ADP-ribosylation of Gs protein
C) ADP-ribosylation of Gi protein
D) Cleavage of SNARE proteins
E) Phospholipase activity
Answer: B) Vibrio cholerae produces cholera toxin, which ADP-ribosylates the Gs protein,
permanently activating it and leading to increased cAMP. This causes massive chloride secretion into
the gut lumen with accompanying water loss, producing the classic "rice water" diarrhea. Pertussis
toxin ADP-ribosylates Gi, Shiga toxin inactivates 60S ribosomes, botulinum/tetanus toxins cleave
SNARE proteins, and alpha toxin has phospholipase activity.
Question 5
, A 55-year-old alcoholic presents with fever, productive cough, and thick red sputum described as
"currant jelly." Chest X-ray shows right upper lobe consolidation. Which bacterial virulence factor is
most important for colonization?
A) Capsule
B) Urease
C) Exotoxin A
D) Protein A
E) M protein
Answer: A) Klebsiella pneumoniae has a prominent polysaccharide capsule that resists phagocytosis
and is associated with the characteristic "currant jelly" sputum. The organism is an important cause
of lobar pneumonia in alcoholics and diabetics. Urease is important for H. pylori, exotoxin A is from
Pseudomonas, Protein A is from S. aureus, and M protein is from S. pyogenes.
Question 6
A 24-year-old sexually active woman presents with purulent vaginal discharge and dysuria. Gram
stain shows Gram-negative diplococci. The organism is oxidase-positive but does not ferment
maltose. What is the most likely diagnosis?
A) Neisseria meningitidis
B) Neisseria gonorrhoeae
C) Haemophilus ducreyi
D) Chlamydia trachomatis
E) Treponema pallidum
Answer: B) Neisseria gonorrhoeae is a Gram-negative diplococcus that is oxidase-positive but does
not ferment maltose (unlike N. meningitidis, which does). It causes purulent urethritis/cervicitis. N.
meningitidis ferments maltose. H. ducreyi causes chancroid, C. trachomatis is not visible on Gram
stain, and T. pallidum requires darkfield microscopy.
Question 7
A 68-year-old diabetic patient develops black necrotic lesions on his face and nose. Biopsy shows
non-septate hyphae branching at wide angles. Which underlying condition predisposed him to this
infection?
Answers with Detailed Rationales Instant Download
EXAM COVERAGE SUMMARY
Bacterial Structure & Staining: Cell wall components (pili, capsules, periplasm), Gram-
positive vs Gram-negative, acid-fast staining, special stains (Giemsa, PAS, Ziehl-Neelsen,
India ink, Silver)
Bacterial Classification & Growth: Aerobic vs anaerobic, obligate intracellular vs
facultative intracellular, encapsulated bacteria, urease-positive, catalase-positive organisms
Bacterial Virulence Factors: Protein A, M protein, IgA protease, Type III secretion system,
exotoxins vs endotoxins (mechanisms, effects, genetics)
Bacterial Toxins: ADP-ribosylating toxins (cholera, pertussis, diphtheria, Pseudomonas
exotoxin A), Shiga toxin, SNARE-cleaving toxins (tetanus, botulinum), superantigens (TSST-
1, streptococcal exotoxin), membrane-disrupting toxins
Specific Bacterial Pathogens: Gram-positive (Staphylococcus, Streptococcus, Bacillus,
Clostridium, Listeria, Corynebacterium, Actinomyces, Nocardia, Mycobacterium), Gram-
negative (Neisseria, Haemophilus, Bordetella, Legionella, Pseudomonas, Enterobacteriaceae,
Vibrio, Campylobacter, Helicobacter, Yersinia, Brucella, Francisella), Spirochetes
(Treponema, Borrelia, Leptospira), Chlamydia, Rickettsia, Mycoplasma
Fungal Pathogens: Systemic mycoses (Histoplasma, Blastomyces, Coccidioides,
Paracoccidioides), Opportunistic fungi (Candida, Aspergillus, Cryptococcus,
Mucor/Rhizopus, Pneumocystis, Sporothrix), Dermatophytes
Parasitic Infections: Protozoa (Giardia, Entamoeba, Cryptosporidium, Toxoplasma,
Plasmodium, Babesia, Trypanosoma, Leishmania, Trichomonas, Naegleria), Helminths
(nematodes, cestodes, trematodes), Ectoparasites
Virology Fundamentals: Viral structure, replication, genetics (recombination, reassortment,
complementation, phenotypic mixing), DNA vs RNA viruses, positive vs negative sense,
segmented vs non-segmented
Specific DNA Viruses: Herpesviruses (HSV-1, HSV-2, VZV, EBV, CMV, HHV-6/7, HHV-
8), Papillomavirus, Polyomavirus (JC, BK), Parvovirus B19, Adenovirus, Poxvirus,
Hepadnavirus (HBV)
Specific RNA Viruses: Picornaviruses, Flaviviruses, Togaviruses, Retroviruses (HIV),
Orthomyxoviruses (Influenza), Paramyxoviruses, Rhabdoviruses (Rabies), Filoviruses
(Ebola), Coronaviruses, Reoviruses (Rotavirus), Hepatitis viruses (HAV, HCV, HDV, HEV)
HIV/AIDS: Pathogenesis, CD4+ count correlation with opportunistic infections, diagnosis,
associated malignancies
Prion Diseases: Pathogenesis, clinical manifestations
Diagnostic Methods: Culture media, staining techniques, serology
Treatment Principles: Antimicrobial agents, resistance mechanisms
Epidemiology & Transmission: Nosocomial infections, food-borne illnesses, vector-borne
diseases
, Host Defense & Immunology: Immune response to infections, vaccines, hypersensitivity
reactions
250 MICROBIOLOGY QUESTIONS
Question 1
A 45-year-old farmer presents with a painless ulcer on his forearm that has developed a black eschar.
He reports handling animal hides. Which virulence factor is most responsible for the characteristic
edematous appearance of this lesion?
A) Tetanospasmin
B) Edema toxin
C) Alpha toxin
D) Shiga toxin
E) Cholera toxin
Answer: B) Bacillus anthracis produces edema toxin that mimics adenylate cyclase, increasing
cAMP and causing localized edema. The black eschar is characteristic of cutaneous anthrax, and the
edema toxin is responsible for the surrounding edematous borders. Tetanospasmin causes spastic
paralysis, alpha toxin causes gas gangrene, Shiga toxin causes dysentery, and cholera toxin causes
watery diarrhea.
Question 2
A 28-year-old woman develops fever, hypotension, and a diffuse erythematous rash during her
menstrual period. She is found to have tampon use. Which bacterial superantigen is most likely
responsible?
A) Streptolysin O
B) Toxic shock syndrome toxin-1
C) Alpha toxin
D) Botulinum toxin
E) Diphtheria toxin
Answer: B) Staphylococcus aureus produces TSST-1, a superantigen that cross-links MHC class II
molecules with T-cell receptors outside the antigen-binding site, causing massive cytokine release (IL-
1, IL-2, IFN-γ, TNF-α) leading to fever, rash, and shock. This is classic toxic shock syndrome
,associated with tampon use. Streptolysin O is from S. pyogenes, alpha toxin from C. perfringens,
botulinum toxin causes flaccid paralysis, and diphtheria toxin inhibits EF-2.
Question 3
A 6-month-old infant presents with a "floppy" appearance, poor feeding, and constipation. The mother
reports feeding the infant honey. Which mechanism best describes the action of the causative toxin?
A) Blocks release of inhibitory neurotransmitters
B) Blocks release of acetylcholine at neuromuscular junctions
C) Inhibits elongation factor-2
D) Activates adenylate cyclase
E) Cleaves IgA
Answer: B) Clostridium botulinum produces botulinum toxin, a protease that cleaves SNARE
proteins, preventing vesicle fusion and release of acetylcholine at neuromuscular junctions. This
causes flaccid paralysis, manifesting as the "floppy baby" syndrome in infants who ingest spores from
honey. Tetanospasmin blocks inhibitory neurotransmitters (GABA/glycine), diphtheria toxin inhibits
EF-2, cholera toxin activates adenylate cyclase, and IgA protease cleaves IgA antibodies.
Question 4
A 32-year-old man who recently returned from a camping trip develops explosive, watery diarrhea
described as "rice water" stools. Which bacterial toxin mechanism is responsible?
A) Inactivation of 60S ribosome
B) ADP-ribosylation of Gs protein
C) ADP-ribosylation of Gi protein
D) Cleavage of SNARE proteins
E) Phospholipase activity
Answer: B) Vibrio cholerae produces cholera toxin, which ADP-ribosylates the Gs protein,
permanently activating it and leading to increased cAMP. This causes massive chloride secretion into
the gut lumen with accompanying water loss, producing the classic "rice water" diarrhea. Pertussis
toxin ADP-ribosylates Gi, Shiga toxin inactivates 60S ribosomes, botulinum/tetanus toxins cleave
SNARE proteins, and alpha toxin has phospholipase activity.
Question 5
, A 55-year-old alcoholic presents with fever, productive cough, and thick red sputum described as
"currant jelly." Chest X-ray shows right upper lobe consolidation. Which bacterial virulence factor is
most important for colonization?
A) Capsule
B) Urease
C) Exotoxin A
D) Protein A
E) M protein
Answer: A) Klebsiella pneumoniae has a prominent polysaccharide capsule that resists phagocytosis
and is associated with the characteristic "currant jelly" sputum. The organism is an important cause
of lobar pneumonia in alcoholics and diabetics. Urease is important for H. pylori, exotoxin A is from
Pseudomonas, Protein A is from S. aureus, and M protein is from S. pyogenes.
Question 6
A 24-year-old sexually active woman presents with purulent vaginal discharge and dysuria. Gram
stain shows Gram-negative diplococci. The organism is oxidase-positive but does not ferment
maltose. What is the most likely diagnosis?
A) Neisseria meningitidis
B) Neisseria gonorrhoeae
C) Haemophilus ducreyi
D) Chlamydia trachomatis
E) Treponema pallidum
Answer: B) Neisseria gonorrhoeae is a Gram-negative diplococcus that is oxidase-positive but does
not ferment maltose (unlike N. meningitidis, which does). It causes purulent urethritis/cervicitis. N.
meningitidis ferments maltose. H. ducreyi causes chancroid, C. trachomatis is not visible on Gram
stain, and T. pallidum requires darkfield microscopy.
Question 7
A 68-year-old diabetic patient develops black necrotic lesions on his face and nose. Biopsy shows
non-septate hyphae branching at wide angles. Which underlying condition predisposed him to this
infection?
