The commonest cause of significant lower gastrointestinal bleed in a middle aged person with unknown reason is
**Core Concept**
The most common cause of significant lower gastrointestinal bleed in a middle-aged person is often related to **diverticulosis**, a condition characterized by the formation of **diverticula** in the wall of the colon. This condition can lead to bleeding due to the erosion of blood vessels within the diverticula.
**Why the Correct Answer is Right**
The correct answer is related to **diverticulosis**, as it is a common condition in middle-aged individuals and can cause significant lower gastrointestinal bleeding. The mechanism involves the rupture of blood vessels in the diverticula, leading to hemorrhage.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because, although it could be a cause, it is not the most common.
**Option B:** Incorrect as it is more commonly associated with upper GI bleeding.
**Option D:** Incorrect because, while it can cause bleeding, it is less common than diverticulosis in this demographic.
**Clinical Pearl / High-Yield Fact**
A key point to remember is that diverticulosis is a common cause of lower GI bleed in middle-aged individuals, and its diagnosis often involves **colonoscopy**.
**Correct Answer:** D. Diverticulosis.