A 56-year-old previously healthy outpatient has 1 day of fever, cough, and pleuritic chest pain. Vitals: T 101.3°F, BP 124/76, HR 96, RR 20, SpO₂ 95% on room air. CXR shows a right lower-lobe infiltrate. No recent antibiotics or comorbidities. Which initial antibiotic regimen is most appropriate?