A 28-year-old man with a 5-year history of ulcerative colitis is admitted with a severe flare of bloody diarrhea. On day 2 of hospitalization he becomes acutely ill with a temperature of 39.1°C, heart rate of 128/min, blood pressure of 96/58 mm Hg, and a distended, tympanitic abdomen with diminished bowel sounds. Abdominal radiograph shows transverse colon dilation to 7 cm. Laboratory studies show a white blood cell count of 18,500/µL, hemoglobin of 9.0 g/dL, and potassium of 3.0 mEq/L. Which of the following is the most appropriate initial management?