## **Core Concept**
The patient presents with acute onset of bright red blood per rectum, which indicates lower gastrointestinal bleeding. Lower GI bleeding is characterized by the presence of bright red blood or clots in the stool, and the source is typically located distal to the ligament of Treitz. The differential diagnosis includes diverticulosis, angiodysplasia, and colorectal cancer.

## **Why the Correct Answer is Right**
The most likely source of bleeding in this patient is **diverticulosis**. Diverticulosis is a condition where small, bulging pouches (diverticula) develop in the digestive tract, most commonly in the sigmoid colon. It is a common cause of lower GI bleeding, especially in older adults. The bleeding is often sudden, painless, and large in volume, as seen in this patient. The patient's age and presentation of painless, large-volume hematochezia (bright red blood per rectum) make diverticulosis a likely cause.

## **Why Each Wrong Option is Incorrect**
* **Option A:** This option is not provided, but typically, other causes such as angiodysplasia or colorectal cancer could be considered.
* **Option B:** If this option suggested a different source such as a rectal source (e.g., hemorrhoids), it would be less likely given the patient's presentation and lab results indicating a more significant bleed.
* **Option C:** If this option suggested an upper GI source, it would be incorrect because the nasogastric lavage was negative for blood or coffee-ground material, making an upper GI source less likely.
* **Option D:** If this option suggested another lower GI source such as inflammatory bowel disease, it would be less likely given the acute presentation and lack of other symptoms such as diarrhea or abdominal pain.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that in cases of lower GI bleeding, a **colonoscopy** is often both diagnostic and therapeutic. For patients with significant bleeding, as in this case, finding and treating the source promptly is critical. Diverticulosis is a common cause of painless, large-volume hematochezia in older adults.

## **Correct Answer:** .