## **Core Concept**
The patient's complaint of black stools, also known as melena, suggests upper gastrointestinal bleeding. This symptom can be associated with various conditions, especially in a patient with a history of coronary artery disease and long-term aspirin use. Aspirin is known to cause gastrointestinal irritation and increase the risk of ulcers and bleeding.

## **Why the Correct Answer is Right**
The correct answer, **C. Gastric ulcer**, is the most probable diagnosis because long-term use of aspirin, a nonsteroidal anti-inflammatory drug (NSAID), can lead to gastric mucosal damage. This damage can progress to gastric ulcers, which are a common cause of upper gastrointestinal bleeding, manifesting as melena (black, tarry stools). The normal abdominal examination does not rule out this diagnosis since the source of bleeding can be within the stomach.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is blank and cannot be evaluated.
- **Option B:** This option is blank and cannot be evaluated.
- **Option D:** This option is blank and cannot be evaluated.

## **Clinical Pearl / High-Yield Fact**
Aspirin and other NSAIDs are well-known to cause gastrointestinal side effects, including ulcers and bleeding, due to their inhibition of cyclooxygenase (COX) enzymes, which protect the gastric mucosa by promoting the production of mucosal prostaglandins. A high-yield fact for exams is that patients on long-term NSAIDs, including low-dose aspirin, are at increased risk of gastrointestinal complications, and this risk can be mitigated with the concomitant use of proton pump inhibitors (PPIs) or other gastroprotective agents.

## **Correct Answer:** C. Gastric ulcer.