In patients over 40 years old, what is the most likely cause of acute lower gastrointestinal bleeding?

In patients over 40 years old, the most likely cause of acute lower gastrointestinal bleeding is diverticulosis. Diverticulosis occurs when small pouches (diverticula) form in the walls of the colon, often due to increased intraluminal pressure. It is particularly common in older adults and typically presents with painless rectal bleeding. The bleeding can result from one of these diverticula eroding into a blood vessel.

While hemorrhoids are another common source of lower gastrointestinal bleeding, especially in older patients, they usually cause external bleeding accompanied by pain and discomfort. In contrast, the bleeding from diverticulosis is usually more significant, can be profuse, and often occurs without accompanying symptoms aside from the bleeding itself.

Crohn's disease and colorectal cancer are also associated with gastrointestinal bleeding, but they are less common causes of acute bleeding specifically in the absence of other symptoms. Crohn's disease often presents with other gastrointestinal symptoms, while colorectal cancer may be more insidious in its onset and bleeding characteristics. Hence, in the context of acute lower gastrointestinal bleeding in patients over 40, diverticulosis stands out as the most prevalent etiology.