Ans. b (1,25,dihydroxy cholecalciferol). (Ref. Harrison's 17th Chapter 346; Ganong 21st/pg.392).*D2 = ergocalciferol, consumed in milk.*D3 = cholecalciferol, formed in sun-exposed skin.*25-OH D3 = storage form.*1,25 (OH)2 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 mmol/L (8.8 mg/dL) prompts a proportional increase in the secretion of parathyroid hormone (PTH) and so mobilizes additional calcium from the bone.# PTH promotes synthesis of l,25(OH)2D 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 25(OH)D level.