A 78-year-old man is brought to the ER with a 12-hour history of constant central abdominal pain, diarrhea, and vomiting. He has a history of atrial fibrillation and was previously treated for congestive heart failure with digoxin. Examination revealed a distended abdomen with tenderness and guarding in the umbilical area and diminished bowel sounds. WBC is 24,000/mL. CT scan reveals edema and thickening of the bowel wall and free fluid around it. What is the most likely diagnosis?