A 5-year-year-old boy presents with a 1-day history of fever (39.0°C), sore throat, and difficulty swallowing. On examination, he has enlarged, erythematous tonsils with exudates and tender cervical lymphadenopathy. A rapid antigen detection test for Group A Streptococcus is positive. He has no known drug allergies. What is the most appropriate antibiotic according to NICE NG84?