What is the most common cause of acute lower gastrointestinal bleeding in patients over the age of 40?

The most common cause of acute lower gastrointestinal bleeding in patients over the age of 40 is diverticulosis. This condition occurs when small pouches (diverticula) form in the wall of the colon and can become inflamed or bleed. In older adults, the presence of diverticulosis is quite prevalent and is often asymptomatic, but when complications like diverticular bleeding occur, they can lead to significant gastrointestinal bleeding.

Diverticulosis is particularly noted for causing hemorrhage because the blood vessels adjacent to these pouches can rupture due to inflammation or increased pressure. The acute nature of such bleeding can result in noticeable symptoms like rectal bleeding and is often significant enough to warrant medical attention.

While hemorrhoids, colorectal cancer, and inflammatory bowel disease are also potential causes of lower gastrointestinal bleeding, diverticulosis stands out as the most common in this age group due to its high prevalence and the mechanism by which it leads to acute bleeding.