A 23-year-old man with a history of ulcerative colitis is admitted to the hospital with bloody diarrhea. His white blood cell count is 13,000/L (1310⁹/L) and hemoglobin is 8.3 g/dL (83 g/L). He is hemodynamically stable. After hospital admission and initiation of intravenous (IV) steroids and infliximab, he undergoes flexible sigmoidoscopy with biopsy. He continues to have 10 to 12 bloody bowel movements daily. Endoscopic and pathologic findings are shown (Figures 1 and 2). What is the best next step in management?