## **Core Concept**
The primary site of production of **gastrin**, a peptide hormone that stimulates the secretion of gastric acid, is a key concept in gastrointestinal physiology. Gastrin plays a crucial role in the digestive process by promoting the release of gastric acid and pepsinogen. The main source of gastrin production needs to be identified among the given options.

## **Why the Correct Answer is Right**
The correct answer, **G cells of the antrum**, is right because G cells, located in the **antrum of the stomach**, are the primary site of gastrin production. These cells release gastrin into the bloodstream in response to food, especially proteins, which then acts on parietal cells in the stomach fundus to stimulate the secretion of gastric acid. This process is a key component of the digestive process, facilitating protein digestion.

## **Why Each Wrong Option is Incorrect**
- **Option A:** The **duodenum** does produce some hormones like secretin and cholecystokinin in response to food, but it is not the primary site of gastrin production.
- **Option B:** The **fundus of the stomach** is primarily involved in the production of gastric acid by parietal cells, not gastrin.
- **Option C:** The **pancreas** produces digestive enzymes and hormones like insulin and glucagon but is not a significant source of gastrin.

## **Clinical Pearl / High-Yield Fact**
A memorable point for exams is that **gastrinomas**, tumors that produce excessive amounts of gastrin, often lead to **Zollinger-Ellison syndrome**, characterized by severe peptic ulcers and high gastric acid secretion. Identifying the primary source of gastrin, like G cells in the antrum, helps in understanding the pathophysiology of such conditions.

## **Correct Answer:** D. G cells of the antrum.