1. A 14-year-old patient is brought in by his parents because of a sore throat. On physical examination, he is febrile, and has pharyngeal erythema with a tonsillar abscess. A throat culture on sheep blood agar yields colonies of gram-positive cocci that are surrounded by a zone of complete hemolysis. The organism was also plated on mannitol salt agar; it grew well and caused the medium to turn yellow. Which of the following microorganisms is the most likely cause of the patient's illness?