**Core Concept**
Gastrinomas are rare neuroendocrine tumors that secrete excessive amounts of gastrin, leading to peptic ulcer disease. The diagnosis of gastrinoma requires a combination of clinical, laboratory, and imaging studies.

**Why the Correct Answer is Right**
The best test for diagnosing gastrinoma is the Secretin Stimulation Test (SST). Secretin, a hormone that inhibits gastric acid secretion, is administered intravenously, and its effect on gastrin and acid secretion is measured. In patients with gastrinoma, secretin stimulates gastrin release, leading to an increase in gastric acid production. This test is more sensitive than fasting gastrin levels and can help differentiate gastrinoma from other causes of hypergastrinemia.

**Why Each Wrong Option is Incorrect**
**Option A:** Fasting gastrin levels are elevated in gastrinoma but are not specific and can be seen in other conditions such as pernicious anemia and chronic atrophic gastritis.
**Option B:** Endoscopy and biopsy are essential for diagnosing gastrinoma but are not definitive tests and may not detect small tumors.
**Option C:** Imaging studies like CT or MRI can help locate gastrinomas but are not the best initial test for diagnosis.

**Clinical Pearl / High-Yield Fact**
Remember that the Secretin Stimulation Test is a key diagnostic tool for gastrinoma, and a positive result can lead to further evaluation and treatment.

**Correct Answer:** B. Endoscopy and biopsy.