This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
[{'content': 'Which of the following is the main site of absorption of vitamin B12? \nSelect and explain. \nA. Duodenum, B. Jejunum, C. Ileum, D. Colon', 'role': 'user'}, {'content': "Answer: C \nExplanation: Most vitamins are absorbed in the upper small intestine, but vitamin B12 is absorbed in the ileum. This vitamin binds to intrinsic factor, a protein secreted by the parietal cells of the stomach, and the complex is absorbed across the ileal mucosa. Vitamin B12 absorption and folate absorption are Na+-independent. Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 26. Digestion, Absorption, & Nutritional Principles. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds), Ganong's Review of Medical Physiology, 24e.", 'role': 'assistant'}]