**Core Concept**
Zollinger Ellison Syndrome (ZES) is a rare disorder characterized by excessive gastrin production, leading to recurrent peptic ulcers and diarrhea. Elevated gastrin levels stimulate an overproduction of gastric acid, causing mucosal damage and ulceration.

**Why the Correct Answer is Right**
The diagnosis of ZES is primarily based on the measurement of serum gastrin levels. A fasting serum gastrin level greater than 200 pg/mL is considered diagnostic of ZES. This is because gastrinomas, the tumors responsible for ZES, produce a gastrin that is resistant to feedback inhibition by acid, leading to an overproduction of gastrin.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because gastrin levels in ZES can be elevated even in the absence of food, making a postprandial measurement unnecessary for diagnosis.

**Option B:** This option is incorrect because the normal fasting gastrin level is less than 200 pg/mL, but values can range from 50-150 pg/mL.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that a gastrin level of 200 pg/mL or higher in a patient with symptoms of peptic ulcer disease should prompt an investigation for ZES, including imaging studies and a secretin stimulation test.

**Correct Answer: C. 200 pg/mL**