Biostatistics & Study Design USMLE Step 1 Practice Question
A chemoprophylaxis regimen is administered to household contacts during a meningococcal outbreak. Baseline vital signs are stable; temperature 37.2°C, HR 88/min, BP 118/76 mmHg. Cerebrospinal fluid cultures from the index case confirm Neisseria meningitidis. The attack rate is 12% in untreated contacts and 3% in treated contacts who received rifampin prophylaxis. No adverse medication reactions are documented. Which of the following best represents the prevented fraction among the treated contacts?
Answer choices
- A9%
- B50%
- C75%Correct answer
- D25%
- E400%
- F3%
See the full explanation
Get the correct-answer rationale, why each distractor is wrong, the underlying mechanism, and high-yield associations — plus adaptive practice that targets your weak areas — with a free MedBoardPRO account.