A 33-year-old previously healthy man with persistent fever and heart murmur is diagnosed with infective endocarditis. He receives a high dosage of a cephalosporin antibiotic during the next 10 days. He now has increasing fatigue. On physical examination, he has tachycardia and scleral icterus. Laboratory studies show a hemoglobin level of 7.5 g/dL, platelet count of 261,000/ mm3, and total WBC count of 8300/ mm3. The direct Coombs test is positive. The peripheral blood smear shows reticulocytosis. Which of the following is the most likely cause of his anemia?