Pneumonia USMLE Step 1 Practice Question
A 6-year-old boy with no significant past medical history presents to the emergency department with a 3-day history of fever (39.5°C), productive cough, and right-sided pleuritic chest pain. Vital signs are: HR 118/min, RR 30/min, BP 108/70 mmHg, SpO2 93% on room air. Physical examination reveals diminished breath sounds and crackles in the right lower lobe. Chest radiograph shows right lower lobe consolidation with a small pleural effusion. Sputum culture grows alpha-hemolytic, optochin-sensitive streptococci. The patient has completed the primary pneumococcal vaccination series (PCV13 at 2, 4, 6, and 12-15 months of age). Which of the following best explains how pneumococcal conjugate vaccine (PCV13) prevents invasive pneumococcal infection in vaccinated individuals?
Answer choices
- AThe vaccine contains live attenuated pneumococcal strains that outcompete pathogenic strains in the nasopharynx
- BThe vaccine provides 100% protection against all serotypes of Streptococcus pneumoniae
- CThe vaccine induces opsonizing antibodies against pneumococcal capsular polysaccharides, enhancing complement-mediated bacteriolysis and phagocytosisCorrect answer
- DThe vaccine prevents bacterial colonization by neutralizing pneumococcal virulence factors in respiratory secretions
- EThe vaccine cross-reacts with cell wall proteins found on all gram-positive bacteria, providing broad protection
- FThe vaccine stimulates T-cell killing of pneumococcal-infected epithelial cells through major histocompatibility complex presentation
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.