**Core Concept**
The patient's presentation of blood-stained stools and abdominal pain suggests a lower gastrointestinal bleeding source, likely from a colonic or rectal origin. This condition often involves a mucosal injury or ulceration, which can be caused by various factors, including ischemia, inflammation, or neoplasia.

**Why the Correct Answer is Right**
The correct answer is likely associated with a condition that involves mucosal injury or ulceration in the lower gastrointestinal tract. One possible cause is **diverticulitis**, which occurs when diverticula (abnormal pouches) in the colon wall become inflamed, leading to mucosal ulceration and bleeding. Diverticulitis is a common cause of lower GI bleeding in older adults, often presenting with abdominal pain and hematochezia (blood-stained stools).

**Why Each Wrong Option is Incorrect**
**Option A:** This option might be incorrect because it does not specifically address the patient's symptoms of blood-stained stools and abdominal pain. A more specific diagnosis or condition would be expected in a differential diagnosis for lower GI bleeding.

**Option B:** This option might be incorrect because it does not directly relate to the patient's symptoms of lower GI bleeding. While constipation can be a symptom of various gastrointestinal conditions, it is not a direct cause of blood-stained stools.

**Option C:** This option might be incorrect because it does not specifically address the patient's symptoms of abdominal pain and blood-stained stools. While a colon cancer diagnosis is possible, it is not the most likely cause of acute lower GI bleeding in this patient.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **lower GI bleeding in older adults often originates from diverticulitis or angiodysplasia**. Diverticulitis is a common cause of lower GI bleeding in this age group, often presenting with abdominal pain and hematochezia.

**Correct Answer:** D. Diverticulitis