A 43-year-old man is seen on an inpatient consultation basis for ulcerative colitis that has been recalcitrant to medical management for the past 2 weeks. He has worsening abdominal pain, persistent diarrhea, rectal bleeding, and intermittent fevers. He was given empiric broad-spectrum antibiotics for elevated leukocytosis. Colonoscopy revealed pancolitis consistent with findings of ulcerative colitis. Although his pain improves, he continues to have 15 to 20 watery bowel movements per day. What is the most appropriate next step in management?