A 29-year-old woman sees her physician because she has had a fever and sore throat for the past 3 days. On physical examination, her temperature is 38.0. The pharynx is erythematous, with yellowish tonsillar °C exudate. Group A Streptococcus pyogenes is cultured. She is treated with ampicillin and recovers fully in 7 days. Two weeks later; she develops fever and a rash and notices a slight decrease in urinary output. Her temperature is 37.7 , and there is a diffuse erythematous rash on the trunk and extremities. Urinalysis shows °C a pH of 6, specific gravity 1.022, l+ proteinuria, l+ hematuria, and no glucose or ketones. Microscopic examination of the urine shows RBCs and WBCs, including eosinophils, but no casts or crystals. Which of the following is the most likely cause of her disease?