This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Misc., Subject name: Physiology
Active form of Vitamin D 
Select and explain. 
A. Cholecalciferol, B. Calciferol, C. Calcitriol, D. 25-hydroxy cholecalciferol
C: Calcitriol
Answer: C: Calcitriol 
Explanation: (C) Calcitriol # Vitamin D Is Metabolized to the Active Metabolite, Calcitriol, in Liver & Kidney.> Cholecalciferol, either synthesized in the skin or from food, undergoes two hydroxylations to yield the active metabolite, 1,25-dihydroxyvitamin D or calcitriol.> Ergocalciferol from fortified foods undergoes similar hydroxylation to yield calcitriol.> In the liver, cholecalciferol is hydroxylated to form the 25-hydroxy-derivative, calcidiol.> This is released into the circula-tion bound to a vitamin D binding globulin, which is the main storage form of the vitamin.> In the kidney, calcidiol undergoes either 1 -hydroxylation to yield the active metabolite 1,25-dihydroxyvitamin D (calcitriol), or 24-hydroxylation to yield a probably inactive metabolite, 24,25-dihydroxyvitamin D (24-hydroxycaicidiol).> Some tissues, other than those that are involved in calcium homeostasis, take up calcidiol from the circulation and synthesize calcitriol that acts within the cell in which it was synthesized.