**Core Concept**
Gastrinoma is a neuroendocrine tumor of the pancreas or duodenum that secretes excessive gastrin, leading to hypergastrinemia. This stimulates gastric acid secretion, causing symptoms related to increased acid production and gastric motility.

**Why the Correct Answer is Right**
Excess gastrin causes prolonged stimulation of parietal cells in the stomach, resulting in hypersecretion of hydrochloric acid. This leads to **increased gastric motility** and **acid-induced diarrhea** due to intestinal irritation and impaired absorption. Diarrhea is a hallmark of Zollinger-Ellison syndrome, which is caused by gastrinoma. The condition is not associated with achlorhydria or ulcers in the typical pattern.

**Why Each Wrong Option is Incorrect**
Option B: Constipation is not seen in gastrinoma; increased gastric motility leads to diarrhea, not reduced motility.
Option C: Achlorhydria is the opposite of what occurs—gastrinoma causes hypersecretion of acid, not deficiency.
Option D: While gastric ulcers may occur, they are typically deep and located in the antrum or body, not superficial, and are not the most characteristic feature. Multiple superficial ulcers are not a hallmark.

**Clinical Pearl / High-Yield Fact**
In Zollinger-Ellison syndrome (caused by gastrinoma), patients present with **severe, recurrent peptic ulcers** and **diarrhea** due to hypersecretion of gastric acid. Remember: **gastrin → acid → diarrhea**.

✓ Correct Answer: A. Diarrhoea