**Core Concept**
Zollinger-Ellison syndrome (ZES) is caused by gastrin-secreting tumors (typically gastrinomas) that lead to excessive gastric acid secretion. Accurate diagnosis requires confirming elevated gastrin levels in the presence of hypersecretion, distinguishing it from other causes of acid hypersecretion.

**Why the Correct Answer is Right**
The secretin stimulation test is the most specific diagnostic tool for ZES. Secretin stimulates gastrin release from the stomach, and in ZES patients, gastrin levels rise significantly despite the presence of a tumor. This test differentiates ZES from other causes of hypergastrinemia (e.g., chronic gastric reflux or hypochlorhydria), where gastrin remains low or normal. It is superior to fasting serum gastrin because it evaluates functional gastrin response, not just baseline levels.

**Why Each Wrong Option is Incorrect**
Option A: Fasting serum gastrin is nonspecific. Elevated levels can occur in conditions like gastric atrophy or chronic gastritis, and may not reflect tumor activity. It lacks sensitivity and specificity.
Option B: CT scan detects tumors but has low sensitivity for small or deep-seated gastrinomas. It may miss early lesions and does not confirm gastrin hypersecretion.
Option C: Endoscopy shows gastric mucosal changes (e.g., ulcers) but cannot diagnose the underlying tumor or confirm hypergastrinemia. It is a supportive test, not diagnostic.

**Clinical Pearl / High-Yield Fact**
In ZES, secretin stimulation test shows a **greater than 2-fold increase in serum gastrin**—a hallmark of disease. A normal response rules out ZES. Always perform this test before considering surgery or treating with proton pump inhibitors.

✓ Correct Answer: D. Secretin stimulation test