**Core Concept**
Zollinger Ellison syndrome (ZES) is a rare disorder characterized by excessive gastric acid secretion due to high levels of gastrin, a hormone produced by the G cells in the stomach. The condition is often caused by a gastrin-secreting tumor, typically a gastrinoma, located in the duodenum or pancreas.

**Why the Correct Answer is Right**
The most important investigation for diagnosing ZES is the **Secretin Stimulation Test**. This test involves administering secretin, a hormone that stimulates gastrin release, to the patient. Blood samples are then taken to measure gastrin levels before and after the secretin injection. In patients with ZES, the gastrin levels will increase significantly in response to secretin, confirming the diagnosis.

**Why Each Wrong Option is Incorrect**
* **Option A:** Endoscopy is not the primary diagnostic tool for ZES, although it may help identify ulcers or other complications of excessive acid secretion.
* **Option B:** Upper GI series (barium swallow) may show ulcers or other structural abnormalities but is not specific for ZES.
* **Option C:** Gastric acid analysis can provide indirect evidence of excessive acid secretion but is not as sensitive or specific as the secretin stimulation test.

**Clinical Pearl / High-Yield Fact**
The secretin stimulation test is the gold standard for diagnosing ZES, as it directly measures the abnormal gastrin response characteristic of this condition.

**Correct Answer:** C. Secretin Stimulation Test