A 72-year-old man presented to the emergency department with an 11-hour history of periumbilical abdominal pain and inability to pass flatus. The pulse was 155 beats per minute, and the blood pressure 83/52 mm Hg. On physical examination, his abdomen was diffusely tender, with the most severe pain in the right upper quadrant. Computed tomography of the abdomen revealed extensive portal venous gas. What is the most common underlying cause?