A 32-year-old man developed a fever and rash over 3 days. Five days later, he has increasing malaise. On physical examination, the maculopapular erythematous rash on his trunk has nearly faded away. His temperature is 37.1degC, and his blood pressure is 135/85 mm Hg. Laboratory studies show a serum creatinine level of 2.8 mg/dL and blood urea nitrogen level of 29 mg/dL. Urinalysis shows 2+ proteinuria; 1+ hematuria; and no glucose, ketones, or nitrite. The leukocyte esterase result is positive. Microscopic examination of urine shows RBCs and WBCs, some of which are eosinophils. Which of the following most likely precipitated his renal disease?