**Core Concept**
Zollinger-Ellison syndrome is a rare disorder characterized by excessive gastric acid production due to excessive gastrin secretion, leading to peptic ulcer disease and diarrhea. This condition is caused by a gastrin-secreting tumor, typically a gastrinoma, which stimulates the parietal cells of the stomach to produce more acid.

**Why the Correct Answer is Right**
The gastrinoma, often found in the duodenum or pancreas, secretes gastrin, a hormone that stimulates the parietal cells in the stomach to produce gastric acid. This increased acid production leads to peptic ulcers and diarrhea. The gastrinoma can be part of a multiple endocrine neoplasia (MEN) syndrome or occur sporadically.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it refers to a different type of tumor that does not produce gastrin, such as a pancreatic neuroendocrine tumor (PNET) that secretes insulin.

**Option B:** This option is incorrect because it refers to a tumor that produces adrenaline (epinephrine), not gastrin, leading to symptoms of pheochromocytoma, such as hypertension and tachycardia.

**Option C:** This option is incorrect because it refers to a tumor that produces ACTH (adrenocorticotropic hormone), leading to Cushing's syndrome, characterized by weight gain, hypertension, and hyperglycemia.

**Clinical Pearl / High-Yield Fact**
The "rule of 10s" for Zollinger-Ellison syndrome is a useful clinical pearl: 10% of patients with gastrinomas have a family history, 10% have multiple endocrine neoplasia (MEN) syndrome, and 10% of patients with MEN syndrome develop gastrinomas.

**Correct Answer: D. Pancreas. Gastrinoma.**