Receptor for vitamin B12-intrinsic factor complex is located in:
**Core Concept**
The vitamin B12-intrinsic factor complex is a crucial pathway for vitamin B12 absorption in the small intestine. The receptor for this complex is a key component in recognizing and binding to the vitamin B12-intrinsic factor complex, enabling its subsequent absorption. This receptor is primarily found in the terminal ileum.
**Why the Correct Answer is Right**
The receptor for the vitamin B12-intrinsic factor complex is the cubilin receptor, also known as megalin. It is a large, endocytic receptor that is expressed on the surface of enterocytes in the terminal ileum. The cubilin receptor recognizes and binds to the vitamin B12-intrinsic factor complex, facilitating its endocytosis and subsequent degradation to release free vitamin B12. This process is essential for the absorption of vitamin B12, which is crucial for the synthesis of DNA, red blood cells, and the maintenance of the nervous system.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the localization of the receptor for the vitamin B12-intrinsic factor complex.
**Option B:** The ileum is the correct location, but it is not specific enough, as the receptor is primarily found in the terminal ileum.
**Option C:** The stomach is not the correct location, as the intrinsic factor is secreted by the parietal cells in the stomach, but the receptor is located in the terminal ileum.
**Clinical Pearl / High-Yield Fact**
The cubilin receptor is also involved in the reabsorption of other nutrients, such as vitamin B9 (folate) and vitamin B7 (biotin). Defects in the cubilin receptor gene can lead to megaloblastic anemia and other metabolic disorders.
**Correct Answer:** C.