A 38-year-old man is evaluated in follow-up after a diagnosis of ulcerative colitis. Ten days ago he was started on prednisone, 60 mg/d, but his symptoms have not improved. He has six to nine bloody bowel movements per day and moderate abdominal pain. He has decreased his oral intake because eating exacerbates his pain and diarrhea.

On physical examination, temperature is 37.0 °C (98.6 °F), blood pressure is 110/56 mm Hg, and pulse rate is 96/min. He is pale but in no distress. The abdomen is diffusely tender without distention, guarding, or rebound.

Laboratory studies reveal a hemoglobin level of 9.7 g/dL (97 g/L) and a leukocyte count of 6300/µL (6.3 × 109/L).

Stool culture and Clostridium difficile assay are negative.

Which of the following is the most appropriate treatment?