**Question:** G-cells are present mostly in

A. Pancreas
B. Liver
C. Stomach
D. Small intestine

**Core Concept:** G-cells are a type of gastrointestinal endocrine cell found in the gastrointestinal tract, particularly in the stomach and small intestine. They are responsible for producing and releasing gastrin, a hormone that regulates gastric acid secretion by stimulating the parietal cells in the stomach.

**Why the Correct Answer is Right:** Gastrin is secreted by G-cells in response to low pH levels in the stomach, promoting acid secretion to digest food and protect the stomach lining. When gastrin is released, it binds to G-protein-coupled receptors on parietal cells, leading to an increase in the production of hydrogen ions (H+) and gastric acid (HCl) secretion into the stomach lumen.

**Why Each Wrong Option is Incorrect:**

A. Pancreas: G-cells are not present in the pancreas. Pancreas is known for its exocrine and endocrine functions, such as insulin and glucagon secretion, not gastrin production.

B. Liver: G-cells are not located in the liver. The liver mainly functions in detoxification, bile production, and blood filtration, not hormone production like gastrin.

C. Stomach (incorrect option): While G-cells are present in the stomach, the correct answer is D, the small intestine, as they are more abundant here and play a more significant role in gastrin secretion.

**Clinical Pearl:** Gastrin regulates gastric acid secretion, which is essential for proper digestion and defense against pathogens. Disturbances in gastrin levels can lead to various gastrointestinal disorders, including gastric ulcers, Zollinger-Ellison syndrome, and hypergastrinemia.

**Correct Answer:** D. Small intestine

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**Why the Correct Answer is Right:** G-cells are predominantly found in the small intestine, particularly in the Brunner's glands, which line the submucosa of the small intestine. These cells secrete gastrin, which stimulates gastric acid secretion from parietal cells in the stomach, aiding in digestion and defense against pathogens.

**Why Each Wrong Option is Incorrect:**

A. Stomach: While G-cells are present in the stomach, they are less abundant and have a more significant role in the small intestine, where they contribute to the regulation of gastric acid secretion and overall gastrointestinal function.

B. Liver: As mentioned earlier, G-cells are not located in the liver. The liver's primary functions include detoxification, bile production, and blood filtration, not gastrin secretion.

C. Gastric mucosa: The question specifically mentions the stomach, which is not the correct answer. G-cells are primarily found in the small intestine and play a more significant role in regulating gastric acid secretion.