A 32-year-old woman with a 6-year history of ulcerative colitis (UC) presents with a 3-week history of bloody diarrhea (8–10 stools/day), abdominal pain, and fever (38.2°C). Laboratory investigations show Hb 9.2 g/dL, WBC 14,500/μL, CRP 8.5 mg/dL, and albumin 2.8 g/dL. Sigmoidoscopy reveals continuous mucosal ulceration with loss of haustra. She is currently on mesalamine monotherapy. What is the most appropriate next step in management?