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?

1. A)The organism's penicillin MIC is at the upper limit of susceptibility, requiring higher serum concentrations than amoxicillin achieves
2. B)Poor penetration of amoxicillin into inflamed middle ear fluid despite in vitro susceptibility of the organismGABARITO
3. C)Selection of a beta-lactamase-producing resistant strain that emerged during treatment
4. D)Inadequate dosing of amoxicillin relative to the penicillin MIC of this isolate
5. E)Reduced oral bioavailability of amoxicillin due to concurrent viral gastroenteritis in this age group