A 23-year-old, ill-appearing woman comes to the emergency room with a fever. She notes the recent development of nausea, diarrhea, and a rash. Her last menstrual period began 3 days ago.Physical examination is remarkable for blood pressure of 90/45 mm Hg and hea rate of 120 beats per minute. A diffuse erythematous rash with areas of desquamation over the hands and feet is noted. Infection with which of the following agents is the most likely cause of these signs and symptoms?