USMLE Step 1 Microbiology and Medical Bacteriology
Practice Exam questions and correct answers– Updated
2026 (Graded A+) instant download pdf
Subject: Microbiology
Subtopic: Bacterial Genetics and Toxin Mechanisms
Question 1: A 4-year-old boy is brought to the emergency department with a 2-day history
of barking cough, inspiratory stridor, and low-grade fever. Physical examination reveals a
grayish-white pseudomembrane firmly adherent to the tonsils and pharynx. The causal
organism produces a virulence factor via lysogenic phage conversion. What is the precise
molecular mechanism of action of this virulence factor?
A) Ribosylation of elongation factor 2 (EF-2)
B) Cleavage of synaptobrevin in motor neurons
C) Adenylate cyclase activation via Gs protein stimulation
D) Inactivation of the 60S ribosomal subunit by removing adenine from rRNA
Correct Answer: A - Ribosylation of elongation factor 2 (EF-2)
Rationale: The clinical vignette describes diphtheria, caused by Corynebacterium diphtheriae.
The diphtheria toxin is encoded by a lysogenic bacteriophage (Corynephage beta). This AB
toxin catalyzes the ADP-ribosylation of elongation factor 2 (EF-2), halting protein synthesis
and causing cell death, which clinically presents as a necrotic pseudomembrane. Choice B
describes tetanus/botulinum toxins. Choice C describes cholera toxin. Choice D describes
Shiga and Shiga-like toxins.
Question 2: An investigator isolates a mutant strain of Streptococcus pneumoniae that lacks
the ability to take up naked DNA from the surrounding environment. This mutant strain most
likely lacks a functional gene encoding for which of the following components?
A) Sex pilus
B) Competence factor
C) Bacteriophage capsid protein
D) Transposase enzyme
Correct Answer: B - Competence factor
Rationale: The uptake of naked DNA from the extracellular environment is known as
transformation. For bacteria to undergo transformation, they must express specific surface
proteins and signaling molecules known as competence factors. Choice A is associated with
,conjugation. Choice C is associated with transduction. Choice D is associated with
transposition.
Question 3: A 24-year-old woman presents to the clinic with painful urination and increased
urinary frequency. Urine culture grows a Gram-negative rod that demonstrates flagellar
motility and is positive for urease production. Which of the following complications is most
directly accelerated by the activity of this organism's urease enzyme?
A) Ascending paralysis
B) Formation of magnesium ammonium phosphate stones
C) Hemolytic uremic syndrome
D) Diffuse intravascular coagulation
Correct Answer: B - Formation of magnesium ammonium phosphate stones
Rationale: The organism described is Proteus mirabilis, a common cause of urinary tract
infections. Urease hydrolyzes urea into ammonia and carbon dioxide, raising the urinary pH.
This alkaline environment precipitates calcium, magnesium, and ammonium phosphate,
leading to the formation of staghorn calculi (struvite stones). Choices A, C, and D are not
associated with urease-producing uropathogens.
Question 4: A 31-year-old male injection drug user is admitted with high fever and a new
holosystolic murmur loudest at the lower left sternal border. Blood cultures grow Gram-
positive cocci in clusters that are catalase-positive and coagulase-positive. The resistance
mechanism of this organism to methicillin is mediated by an altered target site encoded by
which of the following genetic elements?
A) vanA gene cluster
B) mecA gene on the staphylococcal cassette chromosome
C) plasmid-mediated blaZ gene
D) chromosomal gyrA mutation
Correct Answer: B - mecA gene on the staphylococcal cassette chromosome
Rationale: The patient has infective endocarditis caused by Staphylococcus aureus.
Methicillin-resistant S. aureus (MRSA) acquires resistance via the mecA gene, which encodes
penicillin-binding protein 2a (PBP2a). PBP2a has a low affinity for beta-lactam antibiotics.
Choice A is responsible for VRSA/VRE. Choice C mediates standard penicillin resistance via
beta-lactamase production. Choice D mediates fluoroquinolone resistance.
Question 5: A 45-year-old cattle farmer presents with a painless cutaneous ulcer on his right
forearm that has developed a distinctive black, necrotic eschar surrounded by extensive
,gelatinous edema. The causative organism produces a tripartite toxin. Which component of
this toxin acts as a calmodulin-dependent adenylate cyclase?
A) Protective antigen
B) Lethal factor
C) Edema factor
D) Shiga-like toxin
Correct Answer: C - Edema factor
Rationale: The clinical scenario describes cutaneous anthrax caused by Bacillus anthracis. Its
toxin consists of protective antigen (PA), lethal factor (LF), and edema factor (EF). Edema
factor is an intracellular calmodulin-dependent adenylate cyclase that dramatically increases
intracellular cyclic AMP (cAMP) levels, leading to massive fluid extravasation and edema.
Lethal factor (Choice B) is a zinc metalloproteinase that cleaves MAP kinase.
Question 6: A 9-year-old girl is brought to the clinic due to bloody diarrhea and severe
abdominal cramps after eating an undercooked hamburger. She subsequently develops
acute kidney injury, thrombocytopenia, and microangiopathic hemolytic anemia. The toxin
responsible for this condition targets which of the following host cell structures?
A) Elongation factor 2
B) 28S rRNA of the 60S ribosomal subunit
C) Voltage-gated sodium channels
D) Postsynaptic acetylcholine receptors
Correct Answer: B - 28S rRNA of the 60S ribosomal subunit
Rationale: This patient is suffering from Hemolytic Uremic Syndrome (HUS) caused by
Enterohemorrhagic Escherichia coli (EHEC), specifically the O157:H7 serotype. EHEC produces
Shiga-like toxin (verotoxin), which inactivates the 60S ribosomal subunit by cleaving a
specific adenine residue from the 28S rRNA, halting translation and leading to endothelial
cell death in the glomerulus. Choice A is the target of diphtheria and Pseudomonas toxins.
Question 7: A laboratory technician is studying horizontal gene transfer. She observes a
process where a plasmid containing a resistance gene is transferred from a donor bacterium
to a recipient bacterium through direct cell-to-cell contact. This mechanism is completely
inhibited when the solution is treated with a specific substance that targets extracellular
protein extensions. Which transfer mechanism is being observed?
A) Generalized transduction
B) Specialized transduction
, C) F+ Conjugation
D) Natural transformation
Correct Answer: C - F+ Conjugation
Rationale: Direct cell-to-cell contact via a sex pilus (an extracellular protein extension) is
characteristic of conjugation. F+ conjugation involves the transfer of a single strand of
plasmid DNA from an F+ donor to an F- recipient cell. Transduction (Choices A and B) is
mediated by bacteriophages. Transformation (Choice D) involves the uptake of naked DNA
and is sensitive to DNase, not pilus disruption.
Question 8: A 52-year-old male who recently underwent an extensive abdominal surgery
develops a nosocomial urinary tract infection. Culturing reveals Enterococcus faecium that is
highly resistant to vancomycin. What is the specific biochemical modification made by this
organism to evade vancomycin binding?
A) Alteration of D-ala-D-ala to D-ala-D-lac
B) Methylation of the 23S rRNA subunit
C) Acetylation of aminoglycoside binding groups
D) Active efflux of the drug via an ATP-binding cassette transporter
Correct Answer: A - Alteration of D-ala-D-ala to D-ala-D-lac
Rationale: Vancomycin-resistant Enterococcus (VRE) modifies the terminal cell-wall
peptidoglycan precursors from D-alanyl-D-alanine to D-alanyl-D-lactate (or D-alanine-D-
serine). This structural modification causes a loss of hydrogen bonding, decreasing
vancomycin's binding affinity by a factor of 1,000. Choice B describes macrolide resistance.
Choice C describes aminoglycoside resistance.
Subtopic: Virology and Viral Replication Strategies
Question 9: A 28-year-old HIV-positive male with a CD4+ count of 45 cells/uL is diagnosed
with CMV retinitis. He is started on a medication that does not require phosphorylation by
viral kinases to inhibit viral DNA polymerase. Which of the following drugs was most likely
prescribed?
A) Ganciclovir
B) Acyclovir
C) Foscarnet
D) Valacyclovir
Correct Answer: C - Foscarnet
Practice Exam questions and correct answers– Updated
2026 (Graded A+) instant download pdf
Subject: Microbiology
Subtopic: Bacterial Genetics and Toxin Mechanisms
Question 1: A 4-year-old boy is brought to the emergency department with a 2-day history
of barking cough, inspiratory stridor, and low-grade fever. Physical examination reveals a
grayish-white pseudomembrane firmly adherent to the tonsils and pharynx. The causal
organism produces a virulence factor via lysogenic phage conversion. What is the precise
molecular mechanism of action of this virulence factor?
A) Ribosylation of elongation factor 2 (EF-2)
B) Cleavage of synaptobrevin in motor neurons
C) Adenylate cyclase activation via Gs protein stimulation
D) Inactivation of the 60S ribosomal subunit by removing adenine from rRNA
Correct Answer: A - Ribosylation of elongation factor 2 (EF-2)
Rationale: The clinical vignette describes diphtheria, caused by Corynebacterium diphtheriae.
The diphtheria toxin is encoded by a lysogenic bacteriophage (Corynephage beta). This AB
toxin catalyzes the ADP-ribosylation of elongation factor 2 (EF-2), halting protein synthesis
and causing cell death, which clinically presents as a necrotic pseudomembrane. Choice B
describes tetanus/botulinum toxins. Choice C describes cholera toxin. Choice D describes
Shiga and Shiga-like toxins.
Question 2: An investigator isolates a mutant strain of Streptococcus pneumoniae that lacks
the ability to take up naked DNA from the surrounding environment. This mutant strain most
likely lacks a functional gene encoding for which of the following components?
A) Sex pilus
B) Competence factor
C) Bacteriophage capsid protein
D) Transposase enzyme
Correct Answer: B - Competence factor
Rationale: The uptake of naked DNA from the extracellular environment is known as
transformation. For bacteria to undergo transformation, they must express specific surface
proteins and signaling molecules known as competence factors. Choice A is associated with
,conjugation. Choice C is associated with transduction. Choice D is associated with
transposition.
Question 3: A 24-year-old woman presents to the clinic with painful urination and increased
urinary frequency. Urine culture grows a Gram-negative rod that demonstrates flagellar
motility and is positive for urease production. Which of the following complications is most
directly accelerated by the activity of this organism's urease enzyme?
A) Ascending paralysis
B) Formation of magnesium ammonium phosphate stones
C) Hemolytic uremic syndrome
D) Diffuse intravascular coagulation
Correct Answer: B - Formation of magnesium ammonium phosphate stones
Rationale: The organism described is Proteus mirabilis, a common cause of urinary tract
infections. Urease hydrolyzes urea into ammonia and carbon dioxide, raising the urinary pH.
This alkaline environment precipitates calcium, magnesium, and ammonium phosphate,
leading to the formation of staghorn calculi (struvite stones). Choices A, C, and D are not
associated with urease-producing uropathogens.
Question 4: A 31-year-old male injection drug user is admitted with high fever and a new
holosystolic murmur loudest at the lower left sternal border. Blood cultures grow Gram-
positive cocci in clusters that are catalase-positive and coagulase-positive. The resistance
mechanism of this organism to methicillin is mediated by an altered target site encoded by
which of the following genetic elements?
A) vanA gene cluster
B) mecA gene on the staphylococcal cassette chromosome
C) plasmid-mediated blaZ gene
D) chromosomal gyrA mutation
Correct Answer: B - mecA gene on the staphylococcal cassette chromosome
Rationale: The patient has infective endocarditis caused by Staphylococcus aureus.
Methicillin-resistant S. aureus (MRSA) acquires resistance via the mecA gene, which encodes
penicillin-binding protein 2a (PBP2a). PBP2a has a low affinity for beta-lactam antibiotics.
Choice A is responsible for VRSA/VRE. Choice C mediates standard penicillin resistance via
beta-lactamase production. Choice D mediates fluoroquinolone resistance.
Question 5: A 45-year-old cattle farmer presents with a painless cutaneous ulcer on his right
forearm that has developed a distinctive black, necrotic eschar surrounded by extensive
,gelatinous edema. The causative organism produces a tripartite toxin. Which component of
this toxin acts as a calmodulin-dependent adenylate cyclase?
A) Protective antigen
B) Lethal factor
C) Edema factor
D) Shiga-like toxin
Correct Answer: C - Edema factor
Rationale: The clinical scenario describes cutaneous anthrax caused by Bacillus anthracis. Its
toxin consists of protective antigen (PA), lethal factor (LF), and edema factor (EF). Edema
factor is an intracellular calmodulin-dependent adenylate cyclase that dramatically increases
intracellular cyclic AMP (cAMP) levels, leading to massive fluid extravasation and edema.
Lethal factor (Choice B) is a zinc metalloproteinase that cleaves MAP kinase.
Question 6: A 9-year-old girl is brought to the clinic due to bloody diarrhea and severe
abdominal cramps after eating an undercooked hamburger. She subsequently develops
acute kidney injury, thrombocytopenia, and microangiopathic hemolytic anemia. The toxin
responsible for this condition targets which of the following host cell structures?
A) Elongation factor 2
B) 28S rRNA of the 60S ribosomal subunit
C) Voltage-gated sodium channels
D) Postsynaptic acetylcholine receptors
Correct Answer: B - 28S rRNA of the 60S ribosomal subunit
Rationale: This patient is suffering from Hemolytic Uremic Syndrome (HUS) caused by
Enterohemorrhagic Escherichia coli (EHEC), specifically the O157:H7 serotype. EHEC produces
Shiga-like toxin (verotoxin), which inactivates the 60S ribosomal subunit by cleaving a
specific adenine residue from the 28S rRNA, halting translation and leading to endothelial
cell death in the glomerulus. Choice A is the target of diphtheria and Pseudomonas toxins.
Question 7: A laboratory technician is studying horizontal gene transfer. She observes a
process where a plasmid containing a resistance gene is transferred from a donor bacterium
to a recipient bacterium through direct cell-to-cell contact. This mechanism is completely
inhibited when the solution is treated with a specific substance that targets extracellular
protein extensions. Which transfer mechanism is being observed?
A) Generalized transduction
B) Specialized transduction
, C) F+ Conjugation
D) Natural transformation
Correct Answer: C - F+ Conjugation
Rationale: Direct cell-to-cell contact via a sex pilus (an extracellular protein extension) is
characteristic of conjugation. F+ conjugation involves the transfer of a single strand of
plasmid DNA from an F+ donor to an F- recipient cell. Transduction (Choices A and B) is
mediated by bacteriophages. Transformation (Choice D) involves the uptake of naked DNA
and is sensitive to DNase, not pilus disruption.
Question 8: A 52-year-old male who recently underwent an extensive abdominal surgery
develops a nosocomial urinary tract infection. Culturing reveals Enterococcus faecium that is
highly resistant to vancomycin. What is the specific biochemical modification made by this
organism to evade vancomycin binding?
A) Alteration of D-ala-D-ala to D-ala-D-lac
B) Methylation of the 23S rRNA subunit
C) Acetylation of aminoglycoside binding groups
D) Active efflux of the drug via an ATP-binding cassette transporter
Correct Answer: A - Alteration of D-ala-D-ala to D-ala-D-lac
Rationale: Vancomycin-resistant Enterococcus (VRE) modifies the terminal cell-wall
peptidoglycan precursors from D-alanyl-D-alanine to D-alanyl-D-lactate (or D-alanine-D-
serine). This structural modification causes a loss of hydrogen bonding, decreasing
vancomycin's binding affinity by a factor of 1,000. Choice B describes macrolide resistance.
Choice C describes aminoglycoside resistance.
Subtopic: Virology and Viral Replication Strategies
Question 9: A 28-year-old HIV-positive male with a CD4+ count of 45 cells/uL is diagnosed
with CMV retinitis. He is started on a medication that does not require phosphorylation by
viral kinases to inhibit viral DNA polymerase. Which of the following drugs was most likely
prescribed?
A) Ganciclovir
B) Acyclovir
C) Foscarnet
D) Valacyclovir
Correct Answer: C - Foscarnet
