This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Endocrinology, Subject name: Physiology
What is the naturally occurring form of vitamin D in the human skin? 
Select and explain. 
A. Calciferol, B. 7 hydroeholesterol, C. 25 hydroxy cholecalciferol, D. 1,25 dihydroxy cholecalciferol
A: Calciferol
Answer: A: Calciferol 
Explanation: Calciferol is a fat-soluble, crystalline unsaturated alcohol produced by ultraviolet irradiation of ergosterol in plants. It is used as a dietary supplement in the prophylaxis and treatment of rickets, osteomalacia, and other hypocalcemic disorders. Also called ergocalciferol, ole vitamin D2, vitamin D2. See also rickets, viosterol.Ref: Ganong&;s review of medical physiology; 24th edition; page no:-377