A 25-year-old man with ulcerative colitis (UC) is 2 years status post restorative proctocolectomy with J-pouch. He was doing well until 5 weeks ago, when he developed moderate pelvic abdominal pain and a significant increase in the frequency of his bowel movements (>10/day). Dietary changes did not improve the symptoms. Pouchoscopy revealed moderate severity diffuse erythema and friability, with exudates and erosions. Stool cultures are negative for any growth. The best next step in treatment for this patient is