USMLE Step 1 and Microbiology Practice
Exam questions and correct answers–
Updated 2026 (Graded A+) instant download
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Subject: USMLE Step 1 Microbiology
Subtopic: Gram-Positive Cocci and Streptococcal Infections
Question 1: A 28-year-old female presents to the clinic with a sore throat, fever, and cervical
lymphadenopathy. Physical examination reveals tonsillar exudates and a palatal petechiae. A
rapid antigen detection test is positive for group A streptococci. Which of the following
virulence factors is primarily responsible for preventing phagocytosis by binding to the Fc
portion of IgG antibodies?
A) Streptolysin O
B) M protein
C) Hyaluronic acid capsule
D) Streptokinase
Correct Answer: B - M protein
Rationale: The M protein is the primary virulence factor for Streptococcus pyogenes (Group A
Strep). It acts as a superantigen and inhibits phagocytosis by preventing the binding of
complement C3b and, importantly, by binding the Fc portion of IgG antibodies, thereby
preventing opsonization. Streptolysin O is a hemolysin that causes beta-hemolysis and is
immunogenic (leading to ASO titers). Hyaluronic acid capsule is antiphagocytic but acts by
mimicking host tissue (non-immunogenic), not by binding IgG. Streptokinase activates
plasminogen to dissolve clots and facilitate bacterial spread.
Question 2: A 65-year-old male with a history of mitral valve prolapse undergoes a dental
extraction. Two months later, he presents with intermittent fevers, night sweats, and a new
holosystolic murmur. Blood cultures grow gram-positive cocci in chains that are catalase-
negative and optochin-resistant. Which of the following mechanisms best explains the
pathogenesis of this organism’s ability to colonize the heart valve?
A) Production of dextrans from sucrose
B) Secretion of IgA protease
,C) Exotoxin-mediated superantigen activity
D) Biofilm formation via polysaccharide capsule
Correct Answer: A - Production of dextrans from sucrose
Rationale: The clinical presentation is highly suggestive of subacute bacterial endocarditis
caused by Viridans group streptococci (e.g., S. mutans, S. sanguinis). These organisms are
normal flora of the oropharynx. A key virulence factor is the ability to produce extracellular
dextrans from sucrose, which allows them to adhere to fibrin-platelet aggregates on damaged
heart valves. IgA protease is a virulence factor for S. pneumoniae, N. meningitidis, and H.
influenzae. Superantigens are typically associated with S. pyogenes or S. aureus. While biofilms
are important, the synthesis of dextrans is the specific mechanism for initial valve adherence in
Viridans streptococci.
Question 3: A newborn develops signs of sepsis, including respiratory distress and lethargy, 18
hours after birth. Blood cultures demonstrate gram-positive cocci in pairs and chains that exhibit
beta-hemolysis on blood agar. The organism is CAMP-positive and bacitracin-resistant. Which
of the following is the most appropriate prophylactic strategy to have prevented this neonatal
infection?
A) Maternal vaccination against capsular polysaccharide during the third trimester
B) Intrapartum intravenous penicillin or ampicillin
C) Administration of oral erythromycin to the neonate immediately after birth
D) Universal screening of neonates for nasopharyngeal colonization
Correct Answer: B - Intrapartum intravenous penicillin or ampicillin
Rationale: The clinical scenario describes early-onset neonatal sepsis caused by Streptococcus
agalactiae (Group B Streptococcus), which is CAMP-positive and bacitracin-resistant.
Prevention of vertical transmission is achieved by screening pregnant women at 35–37 weeks of
gestation and administering intrapartum antibiotics (penicillin or ampicillin) to those who
screen positive or have other risk factors (e.g., prolonged rupture of membranes, intrapartum
fever). Maternal vaccination for GBS is not a current standard of care. Prophylactic antibiotics
for the neonate are not the standard approach, and screening the neonate is not as effective as
treating the mother.
Subtopic: Gram-Positive Bacilli and Toxin-Mediated Disease
Question 4: A 45-year-old male construction worker presents with a painless black eschar on his
forearm surrounded by significant edema. He reports working with imported animal hides. Gram
stain of the lesion shows large, gram-positive, spore-forming rods. Which of the following
,components of the organism's toxin complex is responsible for the massive edema observed in
the patient?
A) Protective antigen
B) Edema factor
C) Lethal factor
D) Diphtheria toxin
Correct Answer: B - Edema factor
Rationale: The patient has cutaneous anthrax caused by Bacillus anthracis. The anthrax toxin is
a tripartite protein complex. Protective antigen (PA) promotes entry of the other two components
into the cell. Edema factor (EF) is an adenylate cyclase that increases intracellular cAMP,
leading to fluid extravasation and local edema. Lethal factor (LF) is a zinc-dependent
metalloprotease that cleaves MAP kinase, leading to cell death. Diphtheria toxin is produced by
Corynebacterium diphtheriae and inhibits protein synthesis by ADP-ribosylation of EF-2.
Question 5: A 3-year-old unvaccinated child presents with a gray-white pseudomembrane
covering the tonsils and pharynx. The child has a "bull neck" appearance due to cervical
lymphadenopathy. Which of the following is the primary mechanism of action of the causative
agent's potent exotoxin?
A) Cleavage of the 60S ribosomal subunit
B) ADP-ribosylation of elongation factor-2 (EF-2)
C) Overactivation of adenylate cyclase via Gs protein stimulation
D) Inhibition of neurotransmitter release from inhibitory interneurons
Correct Answer: B - ADP-ribosylation of elongation factor-2 (EF-2)
Rationale: The child has diphtheria caused by Corynebacterium diphtheriae. The diphtheria
toxin is an A-B toxin that causes ADP-ribosylation of EF-2, thereby inhibiting protein synthesis
and resulting in cell death. Cleavage of the 60S ribosomal subunit is the mechanism of Shiga
toxin. Overactivation of adenylate cyclase is the mechanism of Cholera toxin and Heat-labile
toxin of E. coli. Inhibition of neurotransmitter release is the mechanism of Tetanospasmin.
Question 6: A 72-year-old patient in a nursing home develops severe, watery diarrhea after a
course of clindamycin for a skin infection. Stool PCR is positive for toxin A and toxin B. Which
of the following is the most likely pathological finding in the colon of this patient?
A) Crypt abscesses
, B) Pseudomembrane formation
C) Granulomatous inflammation
D) Ischemic necrosis
Correct Answer: B - Pseudomembrane formation
Rationale: The clinical scenario describes Clostridioides difficile colitis (pseudomembranous
colitis). Toxin A (enterotoxin) causes fluid secretion and inflammation, while Toxin B (cytotoxin)
causes mucosal damage by depolymerizing actin, leading to the formation of characteristic
pseudomembranes composed of fibrin, leukocytes, and epithelial debris. Crypt abscesses are
more common in ulcerative colitis. Granulomatous inflammation is seen in Crohn disease.
Ischemic necrosis is a separate process involving compromised vascular supply.
Subtopic: Gram-Negative Cocci and Diplococci
Question 7: A 20-year-old male college student presents with high fever, headache, and a
petechial rash on his trunk and lower extremities. A lumbar puncture is performed, and gram-
negative diplococci are visualized within neutrophils. Which of the following virulence factors
allows this organism to survive in the bloodstream and evade the host immune system by
switching surface antigens?
A) IgA protease
B) Pili (fimbriae)
C) Lipooligosaccharide (LOS)
D) Polysaccharide capsule
Correct Answer: B - Pili (fimbriae)
Rationale: The patient has meningococcal meningitis caused by Neisseria meningitidis. While
the polysaccharide capsule is critical for evading phagocytosis and aids in serogrouping, the
organism uses antigenic variation of its pili (fimbriae) to undergo frequent changes in surface
antigens, effectively evading the host’s adaptive immune response. IgA protease facilitates
colonization of the nasopharynx. LOS acts similarly to endotoxin, causing massive inflammatory
response. The capsule is anti-phagocytic but is not the primary factor for antigenic switching.
Question 8: A 25-year-old female presents with purulent vaginal discharge and dysuria. A Gram
stain of the discharge reveals gram-negative diplococci within neutrophils. Which of the
following is the most important reason why an effective vaccine against this organism has not
been developed?
A) Rapid mutation of the capsular polysaccharide
Exam questions and correct answers–
Updated 2026 (Graded A+) instant download
Subject: USMLE Step 1 Microbiology
Subtopic: Gram-Positive Cocci and Streptococcal Infections
Question 1: A 28-year-old female presents to the clinic with a sore throat, fever, and cervical
lymphadenopathy. Physical examination reveals tonsillar exudates and a palatal petechiae. A
rapid antigen detection test is positive for group A streptococci. Which of the following
virulence factors is primarily responsible for preventing phagocytosis by binding to the Fc
portion of IgG antibodies?
A) Streptolysin O
B) M protein
C) Hyaluronic acid capsule
D) Streptokinase
Correct Answer: B - M protein
Rationale: The M protein is the primary virulence factor for Streptococcus pyogenes (Group A
Strep). It acts as a superantigen and inhibits phagocytosis by preventing the binding of
complement C3b and, importantly, by binding the Fc portion of IgG antibodies, thereby
preventing opsonization. Streptolysin O is a hemolysin that causes beta-hemolysis and is
immunogenic (leading to ASO titers). Hyaluronic acid capsule is antiphagocytic but acts by
mimicking host tissue (non-immunogenic), not by binding IgG. Streptokinase activates
plasminogen to dissolve clots and facilitate bacterial spread.
Question 2: A 65-year-old male with a history of mitral valve prolapse undergoes a dental
extraction. Two months later, he presents with intermittent fevers, night sweats, and a new
holosystolic murmur. Blood cultures grow gram-positive cocci in chains that are catalase-
negative and optochin-resistant. Which of the following mechanisms best explains the
pathogenesis of this organism’s ability to colonize the heart valve?
A) Production of dextrans from sucrose
B) Secretion of IgA protease
,C) Exotoxin-mediated superantigen activity
D) Biofilm formation via polysaccharide capsule
Correct Answer: A - Production of dextrans from sucrose
Rationale: The clinical presentation is highly suggestive of subacute bacterial endocarditis
caused by Viridans group streptococci (e.g., S. mutans, S. sanguinis). These organisms are
normal flora of the oropharynx. A key virulence factor is the ability to produce extracellular
dextrans from sucrose, which allows them to adhere to fibrin-platelet aggregates on damaged
heart valves. IgA protease is a virulence factor for S. pneumoniae, N. meningitidis, and H.
influenzae. Superantigens are typically associated with S. pyogenes or S. aureus. While biofilms
are important, the synthesis of dextrans is the specific mechanism for initial valve adherence in
Viridans streptococci.
Question 3: A newborn develops signs of sepsis, including respiratory distress and lethargy, 18
hours after birth. Blood cultures demonstrate gram-positive cocci in pairs and chains that exhibit
beta-hemolysis on blood agar. The organism is CAMP-positive and bacitracin-resistant. Which
of the following is the most appropriate prophylactic strategy to have prevented this neonatal
infection?
A) Maternal vaccination against capsular polysaccharide during the third trimester
B) Intrapartum intravenous penicillin or ampicillin
C) Administration of oral erythromycin to the neonate immediately after birth
D) Universal screening of neonates for nasopharyngeal colonization
Correct Answer: B - Intrapartum intravenous penicillin or ampicillin
Rationale: The clinical scenario describes early-onset neonatal sepsis caused by Streptococcus
agalactiae (Group B Streptococcus), which is CAMP-positive and bacitracin-resistant.
Prevention of vertical transmission is achieved by screening pregnant women at 35–37 weeks of
gestation and administering intrapartum antibiotics (penicillin or ampicillin) to those who
screen positive or have other risk factors (e.g., prolonged rupture of membranes, intrapartum
fever). Maternal vaccination for GBS is not a current standard of care. Prophylactic antibiotics
for the neonate are not the standard approach, and screening the neonate is not as effective as
treating the mother.
Subtopic: Gram-Positive Bacilli and Toxin-Mediated Disease
Question 4: A 45-year-old male construction worker presents with a painless black eschar on his
forearm surrounded by significant edema. He reports working with imported animal hides. Gram
stain of the lesion shows large, gram-positive, spore-forming rods. Which of the following
,components of the organism's toxin complex is responsible for the massive edema observed in
the patient?
A) Protective antigen
B) Edema factor
C) Lethal factor
D) Diphtheria toxin
Correct Answer: B - Edema factor
Rationale: The patient has cutaneous anthrax caused by Bacillus anthracis. The anthrax toxin is
a tripartite protein complex. Protective antigen (PA) promotes entry of the other two components
into the cell. Edema factor (EF) is an adenylate cyclase that increases intracellular cAMP,
leading to fluid extravasation and local edema. Lethal factor (LF) is a zinc-dependent
metalloprotease that cleaves MAP kinase, leading to cell death. Diphtheria toxin is produced by
Corynebacterium diphtheriae and inhibits protein synthesis by ADP-ribosylation of EF-2.
Question 5: A 3-year-old unvaccinated child presents with a gray-white pseudomembrane
covering the tonsils and pharynx. The child has a "bull neck" appearance due to cervical
lymphadenopathy. Which of the following is the primary mechanism of action of the causative
agent's potent exotoxin?
A) Cleavage of the 60S ribosomal subunit
B) ADP-ribosylation of elongation factor-2 (EF-2)
C) Overactivation of adenylate cyclase via Gs protein stimulation
D) Inhibition of neurotransmitter release from inhibitory interneurons
Correct Answer: B - ADP-ribosylation of elongation factor-2 (EF-2)
Rationale: The child has diphtheria caused by Corynebacterium diphtheriae. The diphtheria
toxin is an A-B toxin that causes ADP-ribosylation of EF-2, thereby inhibiting protein synthesis
and resulting in cell death. Cleavage of the 60S ribosomal subunit is the mechanism of Shiga
toxin. Overactivation of adenylate cyclase is the mechanism of Cholera toxin and Heat-labile
toxin of E. coli. Inhibition of neurotransmitter release is the mechanism of Tetanospasmin.
Question 6: A 72-year-old patient in a nursing home develops severe, watery diarrhea after a
course of clindamycin for a skin infection. Stool PCR is positive for toxin A and toxin B. Which
of the following is the most likely pathological finding in the colon of this patient?
A) Crypt abscesses
, B) Pseudomembrane formation
C) Granulomatous inflammation
D) Ischemic necrosis
Correct Answer: B - Pseudomembrane formation
Rationale: The clinical scenario describes Clostridioides difficile colitis (pseudomembranous
colitis). Toxin A (enterotoxin) causes fluid secretion and inflammation, while Toxin B (cytotoxin)
causes mucosal damage by depolymerizing actin, leading to the formation of characteristic
pseudomembranes composed of fibrin, leukocytes, and epithelial debris. Crypt abscesses are
more common in ulcerative colitis. Granulomatous inflammation is seen in Crohn disease.
Ischemic necrosis is a separate process involving compromised vascular supply.
Subtopic: Gram-Negative Cocci and Diplococci
Question 7: A 20-year-old male college student presents with high fever, headache, and a
petechial rash on his trunk and lower extremities. A lumbar puncture is performed, and gram-
negative diplococci are visualized within neutrophils. Which of the following virulence factors
allows this organism to survive in the bloodstream and evade the host immune system by
switching surface antigens?
A) IgA protease
B) Pili (fimbriae)
C) Lipooligosaccharide (LOS)
D) Polysaccharide capsule
Correct Answer: B - Pili (fimbriae)
Rationale: The patient has meningococcal meningitis caused by Neisseria meningitidis. While
the polysaccharide capsule is critical for evading phagocytosis and aids in serogrouping, the
organism uses antigenic variation of its pili (fimbriae) to undergo frequent changes in surface
antigens, effectively evading the host’s adaptive immune response. IgA protease facilitates
colonization of the nasopharynx. LOS acts similarly to endotoxin, causing massive inflammatory
response. The capsule is anti-phagocytic but is not the primary factor for antigenic switching.
Question 8: A 25-year-old female presents with purulent vaginal discharge and dysuria. A Gram
stain of the discharge reveals gram-negative diplococci within neutrophils. Which of the
following is the most important reason why an effective vaccine against this organism has not
been developed?
A) Rapid mutation of the capsular polysaccharide
