An elderly patient with atrial fibrillation (an embolic source), severe central pain, distension, guarding and very high WBC most likely has acute mesenteric ischaemia. The investigation of choice is a CT angiogram of the abdomen, which shows mesenteric arterial narrowing or occlusion, a thickened or non-enhancing bowel wall and pneumatosis. Ultrasound, EUS and upper GI endoscopy cannot assess the mesenteric vessels well enough.