Gastric acid decreased by
**Core Concept**
Gastric acid secretion is regulated by the interplay of various hormones and neural pathways, primarily involving the G cells, D cells, and the vagus nerve. The balance between gastrin, secretin, and somatostatin is crucial in maintaining the optimal level of gastric acid production.
**Why the Correct Answer is Right**
The decrease in gastric acid secretion is primarily mediated by the hormone somatostatin, released by the D cells in the stomach. Somatostatin inhibits the release of gastrin from G cells, thereby reducing the secretion of gastric acid. Additionally, secretin, released by the duodenum in response to acidic chyme, also inhibits gastric acid secretion by stimulating the release of pancreatic bicarbonate and inhibiting gastrin release. The vagus nerve, stimulated by the presence of food in the stomach, releases acetylcholine, which stimulates gastrin release and thus gastric acid secretion.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because gastrin actually stimulates gastric acid secretion, not decreases it.
**Option B:** Incorrect because secretin, while inhibiting gastric acid secretion, is not the primary hormone responsible for the decrease in gastric acid production.
**Option C:** Incorrect because acetylcholine, released by the vagus nerve, stimulates gastrin release and thereby increases gastric acid secretion.
**Clinical Pearl / High-Yield Fact**
In patients with Zollinger-Ellison syndrome, a gastrinoma leads to excessive gastrin production, resulting in severe peptic ulcers and gastric acid hypersecretion. Treatment often involves proton pump inhibitors and surgical resection of the gastrinoma.
**Correct Answer: D. Somatostatin**