Vitamin B12 intrinsic factor absorption occurs in
**Core Concept**
Vitamin B12 absorption requires intrinsic factor, a glycoprotein produced by parietal cells in the stomach. This complex is absorbed in the terminal ileum via active transport, specifically through the cubilin and amnion receptor system.
**Why the Correct Answer is Right**
Intrinsic factor binds vitamin B12 in the stomach and the complex travels to the ileum, where it is absorbed by enterocytes in the terminal ileum. The absorption occurs via the cubilin-megalin receptor pathway in the ileal epithelium. Without intrinsic factor, B12 absorption is severely impaired, leading to pernicious anemia. This process is distinct from other nutrients and occurs only in the ileum.
**Why Each Wrong Option is Incorrect**
Option A: The duodenum absorbs iron and calcium but not vitamin B12 with intrinsic factor. No intrinsic factor binding or absorption occurs here.
Option C: The jejunum is involved in nutrient absorption (e.g., fats, proteins) but lacks the specific receptors for B12-intrinsic factor complexes.
Option D: The colon absorbs water and electrolytes and has no role in B12 absorption.
**Clinical Pearl / High-Yield Fact**
The terminal ileum is the **only** site of vitamin B12 absorption with intrinsic factor. Damage to the ileum (e.g., Crohn’s disease, resection) or lack of intrinsic factor (e.g., pernicious anemia) leads to B12 deficiency.
✓ Correct Answer: B. Ileum