<prompt> Active form of vitamin D is25 - OH - cholecalciferol1- 25 - dihydroxy cholecalciferolErgocalciferolCholecalciferol  <response>: Ans. b 1,25,dihydroxy cholecalciferol. Ref. Harrisons 17th Chapter 346; Ganong 21stpg.392.D2 ergocalciferol, consumed in milk.D3 cholecalciferol, formed in sun-exposed skin.25-OH D3 storage form.1,25 OH2 D3 calcitriol active form.VITAMIN D 1, 25, dihydroxy cholecalciferol is active form of vit D formed from 25hydroxy cholecalciferol in PCT. Since it is a steriod, it acts via receptors steroid receptor superfamily. The mRNAs are produced in response to 1, 25 dihydroxy cholecalciferol diacetate formation of family of calbindinD proteins found in intestine, brain and kidneys, which facilitates Ca movement across intestine and Ca is absorbed. In addition it also facilitates Ca reabsorption in kidneys, increases synthetic activity of osteoblasts and is must for normal calcification of matrix. Vitamin D is synthesized in skin in response to UV radiation and is also absorbed from the diet. It is then transported to the liver, where it undergoes 25-hydroxylation. This metabolite is the major circulating form of vitamin D. The final step in hormone activation, la-hydroxylation, occurs in the kidney. A reduction in serum calcium below 2.2 mmolL 8.8 mgdL prompts a proportional increase in the secretion of parathyroid hormone PTH and so mobilizes additional calcium from the bone. PTH promotes synthesis of l,25OH2D in the kidney, which, in turn, stimulates the mobilization of calcium from bone and intestine and regulates synthesis of PTH by negative feedback. The most specific screening test for vitamin D deficiency is a serum 25OHD level. 