<prompt> Which of the following is the most important vitamin D precursor?Alfa-calcidiol25 Hydroxy-cholecalciferolErgostrol1-25 dihydroxy cholecalciferol  <response>: Ans. b 25 Hydroxy-cholecalciferol. Ref. Ganong, Physiology, 21st392VITAMIN DSource- Vitamin D1, from sun exposure in skin, D2 from plants.- Both converted to 25-OH vit. D in liver and to 1, 25-OH2 vit. D active form in kidney.Function- Increases absorption of dietary calcium.- Increases absorption of dietary phosphate.- Increases bone resorption of Ca2 and PO43.Regulation- Increased PTH causes increased 1, 25-OH2 vit. D production.- Decreased phosphate causes increased l,25-OH, vit. D conversion, l,25OH2 vit. D feedback inhibits its own production. 