A 29-year-old woman has had a fever and sore throat for the past 3 days. On physical examination, her temperature is 38degC. The pharynx is erythematous, with yellowish tonsillar exudate. 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 37.7degC, and there is a diffuse erythematous rash on the trunk and extremities. Urinalysis shows a pH of 6; specific gravity, 1.022; 1+ proteinuria; 1+ hematuria; and no glucose or ketones. Microscopic examination of the urine shows RBCs and WBCs, including eosinophils, but no casts or crystals. What is the most likely cause of her disease?