A 35-year-old woman with Ulcerative Colitis presents with 10 bloody stools per day, fever (38.5°C), tachycardia (110 bpm), and abdominal tenderness. Blood tests show Hb 95 g/L and CRP 80 mg/L. She has been treated with IV Hydrocortisone for 72 hours with no improvement. What is the most appropriate next step?