ESTUDYR
USMLE Step 1 Microbiology Advanced Clinical Integration &
Rapid Recall Master Exam
1.
A 6-year-old child presents with fever, sore throat, and a “sandpaper” rash. Throat culture
reveals beta-hemolytic colonies on blood agar that are bacitracin sensitive. Which virulence
factor is primarily responsible for the rash?
A. Streptolysin O
B. M protein
C. Pyrogenic exotoxin
D. Hyaluronidase
E. Protein A
Correct Answer: C. Pyrogenic exotoxin
Rationale:
Streptococcus pyogenes produces erythrogenic (pyrogenic) exotoxins that act as superantigens,
causing the rash of scarlet fever.
2.
An alcoholic patient develops lobar pneumonia. Gram stain shows lancet-shaped Gram-positive
diplococci. The organism is optochin sensitive and bile soluble. What is the major virulence
factor?
A. IgA protease
B. Polysaccharide capsule
C. Endotoxin
D. M protein
E. Coagulase
Correct Answer: B. Polysaccharide capsule
Rationale:
Streptococcus pneumoniae evades phagocytosis via its polysaccharide capsule, the key virulence
determinant.
,ESTUDYR
3.
A college student living in a dormitory develops meningitis. CSF Gram stain shows Gram-
negative diplococci. Which structural component triggers septic shock?
A. Peptidoglycan
B. Teichoic acid
C. Lipid A
D. Protein A
E. Flagellin
Correct Answer: C. Lipid A
Rationale:
Neisseria meningitidis contains lipooligosaccharide (LOS); the Lipid A portion acts as endotoxin
and triggers TNF-α–mediated septic shock.
4.
A patient presents with pseudomembranous colitis after clindamycin therapy. Which toxin
mechanism causes disease?
A. Inhibits 60S ribosome
B. ADP-ribosylates EF-2
C. Inactivates Rho GTPases
D. Blocks acetylcholine release
E. Activates adenylate cyclase
Correct Answer: C. Inactivates Rho GTPases
Rationale:
Clostridioides difficile toxin A and B glucosylate Rho proteins, disrupting cytoskeleton and tight
junctions.
5.
An IV drug user develops tricuspid endocarditis. Blood cultures grow Gram-positive cocci in
clusters, catalase-positive, coagulase-positive. Which factor allows fibrin clot formation?
A. Streptokinase
B. Coagulase
C. IgA protease
, ESTUDYR
D. DNase
E. M protein
Correct Answer: B. Coagulase
Rationale:
Staphylococcus aureus produces coagulase, converting fibrinogen to fibrin, protecting bacteria
from immune clearance.
6.
A neonate develops meningitis. The organism is beta-hemolytic, bacitracin-resistant, and CAMP
test positive. What is the organism?
A. Streptococcus pyogenes
B. Streptococcus pneumoniae
C. Group B Streptococcus
D. Enterococcus faecalis
E. Listeria monocytogenes
Correct Answer: C. Group B Streptococcus
Rationale:
Streptococcus agalactiae is CAMP-positive and a major cause of neonatal meningitis.
7.
A patient with cavitary lung lesions has acid-fast bacilli on Ziehl–Neelsen stain. Which feature
explains acid-fastness?
A. Lipopolysaccharide
B. Mycolic acid
C. Capsule
D. Teichoic acid
E. Spores
Correct Answer: B. Mycolic acid
Rationale:
Mycobacterium tuberculosis has mycolic acids in its cell wall, retaining carbol fuchsin dye.
USMLE Step 1 Microbiology Advanced Clinical Integration &
Rapid Recall Master Exam
1.
A 6-year-old child presents with fever, sore throat, and a “sandpaper” rash. Throat culture
reveals beta-hemolytic colonies on blood agar that are bacitracin sensitive. Which virulence
factor is primarily responsible for the rash?
A. Streptolysin O
B. M protein
C. Pyrogenic exotoxin
D. Hyaluronidase
E. Protein A
Correct Answer: C. Pyrogenic exotoxin
Rationale:
Streptococcus pyogenes produces erythrogenic (pyrogenic) exotoxins that act as superantigens,
causing the rash of scarlet fever.
2.
An alcoholic patient develops lobar pneumonia. Gram stain shows lancet-shaped Gram-positive
diplococci. The organism is optochin sensitive and bile soluble. What is the major virulence
factor?
A. IgA protease
B. Polysaccharide capsule
C. Endotoxin
D. M protein
E. Coagulase
Correct Answer: B. Polysaccharide capsule
Rationale:
Streptococcus pneumoniae evades phagocytosis via its polysaccharide capsule, the key virulence
determinant.
,ESTUDYR
3.
A college student living in a dormitory develops meningitis. CSF Gram stain shows Gram-
negative diplococci. Which structural component triggers septic shock?
A. Peptidoglycan
B. Teichoic acid
C. Lipid A
D. Protein A
E. Flagellin
Correct Answer: C. Lipid A
Rationale:
Neisseria meningitidis contains lipooligosaccharide (LOS); the Lipid A portion acts as endotoxin
and triggers TNF-α–mediated septic shock.
4.
A patient presents with pseudomembranous colitis after clindamycin therapy. Which toxin
mechanism causes disease?
A. Inhibits 60S ribosome
B. ADP-ribosylates EF-2
C. Inactivates Rho GTPases
D. Blocks acetylcholine release
E. Activates adenylate cyclase
Correct Answer: C. Inactivates Rho GTPases
Rationale:
Clostridioides difficile toxin A and B glucosylate Rho proteins, disrupting cytoskeleton and tight
junctions.
5.
An IV drug user develops tricuspid endocarditis. Blood cultures grow Gram-positive cocci in
clusters, catalase-positive, coagulase-positive. Which factor allows fibrin clot formation?
A. Streptokinase
B. Coagulase
C. IgA protease
, ESTUDYR
D. DNase
E. M protein
Correct Answer: B. Coagulase
Rationale:
Staphylococcus aureus produces coagulase, converting fibrinogen to fibrin, protecting bacteria
from immune clearance.
6.
A neonate develops meningitis. The organism is beta-hemolytic, bacitracin-resistant, and CAMP
test positive. What is the organism?
A. Streptococcus pyogenes
B. Streptococcus pneumoniae
C. Group B Streptococcus
D. Enterococcus faecalis
E. Listeria monocytogenes
Correct Answer: C. Group B Streptococcus
Rationale:
Streptococcus agalactiae is CAMP-positive and a major cause of neonatal meningitis.
7.
A patient with cavitary lung lesions has acid-fast bacilli on Ziehl–Neelsen stain. Which feature
explains acid-fastness?
A. Lipopolysaccharide
B. Mycolic acid
C. Capsule
D. Teichoic acid
E. Spores
Correct Answer: B. Mycolic acid
Rationale:
Mycobacterium tuberculosis has mycolic acids in its cell wall, retaining carbol fuchsin dye.
