This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: vitamins, Subject name: Biochemistry
Vitamin also acting as hormone 
Select and explain. 
A. Vitamin D, B. Vitamin A, C. Vitamin B1, D. Vitamin C
A: Vitamin D
Answer: A: Vitamin D 
Explanation: Vitamin D is not strictly a vitamin since it can be synthesized in the skin the dietary source is required only when sunlight exposure is inadequate Its main function is in the regulation of calcium absorption and homeostasis most of the actions of vitD mediated by nuclear receptors. ref harpers biochemistry 30th ed page 551