Vitamin B12 and Folate Quiz

See Section. Option E is correct. The terminal ileum, the final part of the small intestine, contains receptors that take up the intrinsic factor-B12 complex. Intrinsic factor is made by gastric parietal cells, but its site of production is not the site of B12 absorption. After B12 binds this protein, the complex travels along the small intestine before reaching the uptake receptors. It enters enterocytes, the cells lining the intestine, through receptor-mediated endocytosis, meaning uptake into the cell after binding a surface receptor. Disease or removal of the terminal ileum can therefore impair this pathway and cause B12 deficiency. Remember the distinction: the stomach supplies the carrier and the ileum receives its cargo.

The duodenum (D) is where enzymes free B12 from haptocorrin so it can bind intrinsic factor, rather than the main uptake site for that complex. Jejunal transport with bile salts (C) confuses B12 handling with the role of bile salts in fat absorption. Parietal cells in the stomach (B) secrete intrinsic factor rather than absorb its B12 complex. Colonic bacterial products (A) do not make the colon the principal physiological absorption site.