USMLE: Microbiology Exam Questions with Correct
Verified Solutions 100% 2024/2025
Question 1: Bacterial Pathogenesis
Question: A 32-year-old man presents with a fever, headache, and a rash that started on his
wrists and ankles and has spread to his trunk. On examination, he has a history of recent camping
in a wooded area. The patient's rash is described as petechial. What is the most likely causative
organism?
A) Rickettsia rickettsii
B) Borrelia burgdorferi
C) Chlamydia trachomatis
D) Mycoplasma pneumoniae
Answer: A) Rickettsia rickettsii
Explanation: The patient's symptoms and rash are consistent with Rocky Mountain spotted
fever, which is caused by Rickettsia rickettsii. The rash typically starts on the wrists and ankles
and spreads centrally, often becoming petechial. This is associated with recent exposure to
wooded areas, where the vector, the tick, is commonly found.
Question 2: Antibiotic Mechanism of Action
Question: Which of the following antibiotics inhibits bacterial protein synthesis by binding to
the 50S ribosomal subunit?
A) Penicillin
B) Vancomycin
C) Erythromycin
D) Ciprofloxacin
Answer: C) Erythromycin
Explanation: Erythromycin is a macrolide antibiotic that binds to the 50S ribosomal subunit,
inhibiting bacterial protein synthesis. Penicillin targets cell wall synthesis, vancomycin also
targets cell wall synthesis, and ciprofloxacin inhibits DNA gyrase, which is important for DNA
replication.
Question 3: Viral Classification
Question: A 45-year-old woman presents with severe pharyngitis and fever. A rapid test for
group A Streptococcus is negative. A throat culture grows small, round colonies with a
characteristic greenish halo on blood agar. What is the most likely causative agent?
Verified Solutions 100% 2024/2025
Question 1: Bacterial Pathogenesis
Question: A 32-year-old man presents with a fever, headache, and a rash that started on his
wrists and ankles and has spread to his trunk. On examination, he has a history of recent camping
in a wooded area. The patient's rash is described as petechial. What is the most likely causative
organism?
A) Rickettsia rickettsii
B) Borrelia burgdorferi
C) Chlamydia trachomatis
D) Mycoplasma pneumoniae
Answer: A) Rickettsia rickettsii
Explanation: The patient's symptoms and rash are consistent with Rocky Mountain spotted
fever, which is caused by Rickettsia rickettsii. The rash typically starts on the wrists and ankles
and spreads centrally, often becoming petechial. This is associated with recent exposure to
wooded areas, where the vector, the tick, is commonly found.
Question 2: Antibiotic Mechanism of Action
Question: Which of the following antibiotics inhibits bacterial protein synthesis by binding to
the 50S ribosomal subunit?
A) Penicillin
B) Vancomycin
C) Erythromycin
D) Ciprofloxacin
Answer: C) Erythromycin
Explanation: Erythromycin is a macrolide antibiotic that binds to the 50S ribosomal subunit,
inhibiting bacterial protein synthesis. Penicillin targets cell wall synthesis, vancomycin also
targets cell wall synthesis, and ciprofloxacin inhibits DNA gyrase, which is important for DNA
replication.
Question 3: Viral Classification
Question: A 45-year-old woman presents with severe pharyngitis and fever. A rapid test for
group A Streptococcus is negative. A throat culture grows small, round colonies with a
characteristic greenish halo on blood agar. What is the most likely causative agent?
