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The most likely diagnosis for a 72-year-old man presenting with severe diffuse abdominal pain and bloody mucus during a digital rectal exam is acute mesenteric ischemia. This condition results from a reduction of blood flow to the intestines, which can occur due to various factors such as embolism, thrombosis, or vascular occlusion.

In patients with acute mesenteric ischemia, the abdominal pain is often severe and disproportionate to the physical examination findings, which is characteristic of this condition. Additionally, the presence of bloody stool or mucus can occur due to intestinal necrosis as the ischemic bowel tissue may slough off.

The demographic of an older adult, combined with the symptoms described, aligns well with acute mesenteric ischemia, which is more prevalent in this population due to factors like atherosclerosis leading to compromised blood flow. In contrast, while conditions such as appendicitis, diverticulitis, and pancreatitis can cause abdominal pain, they typically present with different symptom patterns, and bloody mucus is not a predominant feature in those diagnoses.