Antimicrobials USMLE Step 1 Practice Question
A 6-year-old boy presents with a 5-day history of otalgia, fever (38.9°C), and purulent ear drainage despite appropriate oral amoxicillin therapy (25 mg/kg/day divided twice daily). The parents confirm good compliance with medication administration. Vital signs are HR 110/min, RR 24/min, BP 105/68 mmHg. Otoscopy reveals purulent drainage from the ear canal. Tympanometry shows Type B tracing consistent with middle ear effusion. Tympanic membrane appears intact without perforation. Culture of ear drainage grows Streptococcus pneumoniae with a penicillin MIC of 0.5 mcg/mL (susceptible). Which of the following best explains the clinical failure of amoxicillin therapy in this patient?
Answer choices
- AThe organism's penicillin MIC is at the upper limit of susceptibility, requiring higher serum concentrations than amoxicillin achieves
- BPoor penetration of amoxicillin into inflamed middle ear fluid despite in vitro susceptibility of the organismCorrect answer
- CSelection of a beta-lactamase-producing resistant strain that emerged during treatment
- DInadequate dosing of amoxicillin relative to the penicillin MIC of this isolate
- EReduced oral bioavailability of amoxicillin due to concurrent viral gastroenteritis in this age group
- FDevelopment of an abscess localized to the mastoid bone preventing drug penetration
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