Antimicrobials USMLE Step 1 Practice Question
A 28-year-old man presents to the emergency department with a 2-day history of fever, severe headache, neck stiffness, and photophobia. Vital signs show temperature 39.8°C, blood pressure 118/74 mmHg, and heart rate 108/min. Physical examination reveals positive Kernig and Brudzinski signs. Lumbar puncture shows cloudy cerebrospinal fluid (CSF) with opening pressure 280 mmH₂O, white blood cell count 1,200 cells/mm³ (95% neutrophils), protein 180 mg/dL, and glucose 28 mg/dL (serum glucose 95 mg/dL). Empiric antimicrobial therapy is initiated with ceftriaxone 2 g IV every 12 hours, vancomycin 15-20 mg/kg IV every 8-12 hours, and dexamethasone. CSF culture subsequently grows Streptococcus pneumoniae sensitive to penicillin (MIC 0.06 mcg/mL). Blood cultures remain negative. Which of the following best explains the clinical rationale for including vancomycin in the empiric regimen for bacterial meningitis in this patient?
Answer choices
- AVancomycin achieves higher absolute concentrations in CSF than ceftriaxone due to superior lipophilicity and blood-brain barrier penetration
- BCeftriaxone monotherapy has inadequate CSF penetration and requires a synergistic agent to achieve bactericidal activity
- CThe prevalence of penicillin-resistant and cephalosporin-resistant S. pneumoniae strains necessitates empiric dual therapy until susceptibilities are knownCorrect answer
- DVancomycin provides coverage against gram-negative organisms such as Neisseria meningitidis that cephalosporins cannot reach in the meninges
- EEarly vancomycin administration reduces CSF inflammation and dexamethasone requirement in meningitis patients
- FVancomycin prevents the development of antibiotic resistance by inhibiting beta-lactamase production in S. pneumoniae
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