**Core Concept**
Gastrinoma is a tumor of the pancreas or duodenum that secretes excessive amounts of gastrin, leading to peptic ulcer disease and other complications. The underlying principle being tested is the pathophysiology of gastrinoma and its effects on the gastrointestinal system.

**Why the Correct Answer is Right**
Gastrinoma leads to excessive gastrin secretion, which in turn causes increased gastric acid production. This results in peptic ulcers, diarrhea, and weight loss. The mechanism involves the stimulation of parietal cells in the stomach to produce more gastric acid, leading to mucosal damage and ulceration. The increased gastrin levels also cause hypertrophy of the gastric mucosa.

**Why Each Wrong Option is Incorrect**
**Option A:** Diarrhea is a common symptom of gastrinoma due to the increased gastric acid production, which can lead to malabsorption of nutrients.
**Option B:** Weight loss is a result of malabsorption and chronic inflammation caused by the peptic ulcers.
**Option C:** Not occurring in gastrinoma; this option is actually incorrect.

**Clinical Pearl / High-Yield Fact**
Gastrinoma is a classic cause of Zollinger-Ellison syndrome, characterized by multiple peptic ulcers, diarrhea, and severe gastric acid hypersecretion. This condition requires aggressive treatment with proton pump inhibitors and surgical resection of the tumor.

**Correct Answer: C. Not occurring in gastrinoma.**