A 5-year-old boy is brought to the GP with a 1-week history of a sore throat, low-grade fever, and general malaise. On examination, his tonsils are erythematous and enlarged with some exudate. Cervical lymph nodes are palpable and tender. A rapid antigen detection test (RADT) for Group A Streptococcus is positive. He has no history of penicillin allergy. According to NICE NG84, what is the most appropriate antibiotic to prescribe?