**Question:** All of the following have inhibitory effect on secretion of gastrin, except

A. Histamine (H2-receptor antagonists)
B. Secretin
C. Cholecystokinin (CCK)
D. Gastrin itself

**Core Concept:**

Gastrin is a hormone primarily produced by the G cells in the antrum of the stomach. It plays a crucial role in regulating gastric acid secretion through its interaction with parietal cells in the stomach epithelium. Gastrin secretion is under the influence of various factors, including the presence of food, acidity of the stomach, and somatostatin. Somatostatin is a peptide hormone that acts as a suppressor of gastrin and other gastrointestinal hormones.

**Why the Correct Answer is Right:**

Option D (Gastrin itself) is the correct answer because gastrin is the hormone that stimulates its own secretion. This positive feedback loop ensures a rapid increase in gastrin levels in response to the presence of food in the stomach, leading to increased gastric acid secretion.

**Why Each Wrong Option is Incorrect:**

Option A (Histamine) and its antagonists (e.g., Ranitidine, Cimetidine) suppress gastrin secretion. Histamine, released from enterochromaffin-like (ECL) cells, stimulates gastrin release through H2-receptors. H2-receptor antagonists block this effect, thereby reducing gastrin secretion.

Option B (Secretin) is incorrect because secretin is a hormone released from the sphincter of Oddi in response to the presence of bile and pancreatic enzymes in the duodenum. Secretin stimulates the pancreas to secrete bicarbonate-rich pancreatic juice, which helps to neutralize the acidic chyme entering the duodenum. Secretin does not have an inhibitory effect on gastrin secretion.

Option C (Cholecystokinin, also known as CCK) is incorrect because cholecystokinin is a hormone released from the duodenum in response to the presence of fats, proteins, and bile in the duodenum. Cholecystokinin stimulates the pancreas to secrete enzymes and bicarbonate-rich pancreatic juice, which aids in the digestion of fats, proteins, and carbohydrates. Cholecystokinin does not have an inhibitory effect on gastrin secretion.

**Clinical Pearl:**

In clinical practice, understanding the interplay between gastrin, histamine, and secretin is essential for managing acid-related disorders, such as gastroesophageal reflux disease (GERD) and peptic ulcer disease. Treating GERD with histamine H2-receptor antagonists reduces gastric acid secretion, decreasing gastrin release and potentially worsening GERD symptoms in some cases. Treating peptic ulcer disease with somatostatin analogs (e.g., octreotide) inhibits gastrin release, but the treatment duration should be carefully monitored to avoid exacerbating GERD symptoms.

**Explanation of the Question:**

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