## **Core Concept**
Cholecystokinin (CCK) is a peptide hormone of the gastrointestinal system responsible for stimulating the digestion of fat and protein. It is produced in response to food, particularly fats and proteins, in the duodenum. The primary source of CCK production is crucial for understanding its role in digestion.

## **Why the Correct Answer is Right**
The correct answer, **Duodenum**, is right because CCK is primarily produced by the **I cells (also known as cholecystokinin cells)** located in the mucosa of the duodenum. When these cells are stimulated by the presence of nutrients, especially fats and proteins, they release CCK into the bloodstream. CCK then acts on the gallbladder to release bile and on the pancreas to release digestive enzymes.

## **Why Each Wrong Option is Incorrect**
- **Option A: Stomach** - This is incorrect because, although the stomach secretes digestive enzymes and acids, it is not the primary site for CCK production. The stomach does release gastrin, another hormone involved in digestion.
- **Option B: Liver** - This is incorrect because the liver produces bile but is not directly involved in CCK production. The liver does respond to CCK by signaling the gallbladder to release bile.
- **Option C: Jejunum** - This is incorrect because, while the jejunum is part of the small intestine and involved in nutrient absorption, it is not the primary site for CCK production. CCK production mainly occurs in the duodenum.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that cholecystokinin plays a significant role in facilitating digestion by coordinating the release of digestive enzymes and bile. Its release is stimulated by the presence of food, especially fats and proteins, in the duodenum. This hormone is a critical component of the digestive process, making its site of production, the duodenum, a high-yield fact for exams.

## **Correct Answer:** D. Duodenum