## **Core Concept**
The intrinsic factor of Castle, also known simply as intrinsic factor, is a glycoprotein in the stomach that plays a crucial role in the absorption of vitamin B12. It binds to vitamin B12 from food and facilitates its absorption in the ileum of the small intestine. This process is essential for the synthesis of DNA and the maintenance of the nervous system.

## **Why the Correct Answer is Right**
The correct answer, **parietal cells**, is right because intrinsic factor is produced and secreted by these cells, which are located in the gastric glands of the stomach lining. Parietal cells are responsible for secreting not only intrinsic factor but also gastric acid (hydrochloric acid) and gastric enzymes. The intrinsic factor-vitamin B12 complex is recognized by specific receptors in the ileum, allowing for the uptake of vitamin B12.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Chief cells, also known as peptic cells or gastric chief cells, primarily secrete pepsinogen, the inactive form of pepsin, which breaks down proteins into smaller peptides. They do not produce intrinsic factor.
- **Option B:** Enterochromaffin-like cells (ECL cells) release histamine in response to gastrin stimulation, which in turn stimulates parietal cells to secrete hydrochloric acid. They are not involved in the secretion of intrinsic factor.
- **Option C:** Mucous neck cells and surface mucous cells produce mucus that protects the stomach lining from acid and enzymes. They are not responsible for intrinsic factor production.

## **Clinical Pearl / High-Yield Fact**
A key clinical correlation to remember is that a deficiency in intrinsic factor leads to pernicious anemia, a condition characterized by a lack of vitamin B12 absorption, resulting in megaloblastic anemia and neurological problems. This condition can be treated with vitamin B12 injections or oral supplements.

## **Correct Answer:** D. Parietal cells.