A yоung pаrent brings а 4-yeаr-оld child in tо the pediatric clinic with a mild fever and a red, spotty rash that is beginning to fade. The child's heart rate is rapid, and the rhythm is abnormal. The parent states the child has been healthy until about 3 weeks ago when the child had a sore throat. The nurse suspects rheumatic carditis. What organism causes rheumatic carditis?