A 5-year-old girl presents with a 3-day history of fever, sore throat, and difficulty swallowing. On examination, she has enlarged, erythematous tonsils with white exudates, tender cervical lymphadenopathy, and palatal petechiae. A rapid antigen detection test for Group A Streptococcus is positive. She has no history of penicillin allergy. What is the most appropriate antibiotic treatment?