**Core Concept**
Diverticulosis is a condition characterized by the formation of diverticula, small pouches in the wall of the colon, which can lead to complications such as diverticulitis, bleeding, or obstruction. In older adults, diverticulosis is a common cause of lower gastrointestinal bleeding, often presenting as massive bleeding per rectum.

**Why the Correct Answer is Right**
The formation of diverticula in diverticulosis occurs due to increased pressure within the colonic lumen, which is often a result of a low-fiber diet and decreased physical activity. As the pressure increases, the weak points in the colonic wall, such as the vasa recta, can rupture, leading to bleeding. In a 70-year-old patient, the likelihood of diverticulosis causing massive bleeding per rectum is higher due to the increased prevalence of this condition with age.

**Why Each Wrong Option is Incorrect**

**Option B:** Carcinoma colon, while a potential cause of lower gastrointestinal bleeding, is less common in older adults and typically presents with other symptoms such as weight loss, abdominal pain, or changes in bowel habits.

**Option C:** Colitis, including inflammatory bowel disease (IBD) and infectious colitis, can cause lower gastrointestinal bleeding but is less likely to present as massive bleeding per rectum in an older adult.

**Option D:** Polyps, while a potential cause of lower gastrointestinal bleeding, are typically smaller and less likely to cause massive bleeding per rectum compared to diverticulosis.

**Clinical Pearl / High-Yield Fact**
Diverticulosis is a common condition in older adults, and its prevalence increases with age. It is essential to consider diverticulosis as a potential cause of lower gastrointestinal bleeding in this population, especially when other symptoms are absent.

**Correct Answer:**
✓ Correct Answer: A. Diverticulosis.