A 32-year-old man with a 6-year history of ulcerative colitis presents with a 3-week history of bloody diarrhea (8–10 stools/day), abdominal cramping, and fever (38.5°C). Laboratory investigations show Hb 9.2 g/dL, WBC 14,500/µL, ESR 68 mm/h, and CRP 12 mg/dL. Stool culture and C. difficile toxin are negative. Flexible sigmoidoscopy reveals continuous mucosal ulceration with friability and loss of haustra from rectum to mid-colon. What is the most appropriate next step in management?