A 51-year-old man with a 10-year history of ulcerative colitis presents with more than 10 bloody stools per day, temperature of 100.4°F (38°C), pulse of 110 beats/min, and colonic distention on radiography with a transverse colon diameter of 6 cm. Laboratory findings include hemoglobin of 9.5 g/dL (95 g/L), erythrocyte sedimentation rate of 55 mm/h, and albumin of 2.3 g/dL (23 g/L). The patient’s abdomen is distended and diffusely tender. After 48 hours of medical treatment with steroids and broad-spectrum antibiotics, the patient’s condition remains unchanged. What is the most appropriate next step?