An 8 year old boy is brought to the emergency room with a 3-day history of fever to 102 degrees F and abdominal pain. He also complains of pain in his right knee and right elbow. He was seen four weeks ago because of a sore throat and a rash. A throat culture performed at that time grew gram-positive cocci in chains. Amoxicillin was prescribed, but the boy's mother did not fill the prescription. On physical exam his temp is 101.7F, HR 96, and BP 100/60. Cardiac exam reveals a pansystolic blowing murmur heard best at the apex. His right elbow is tender on extension and flexion with mild swelling. Laboratory tests reveal a positive C-reactive protein, an ESR of 40 and a WBC of 22,000 with a left shift. ECG shows a prolonged PR interval. Which of the following tests would be positive for the microorganism responsible for this patient's illness?