A 28-year-old woman presents to the emergency department with a 2-day history of severe sore throat, fever (39.5°C), and difficulty swallowing. On examination, she has exudative pharyngitis with enlarged tonsils, palatal petechiae, and tender anterior cervical lymphadenopathy. A rapid antigen detection test (RADT) for group A Streptococcus is positive. She has no history of penicillin allergy. What is the most appropriate immediate antibiotic therapy?