In chronic renal failure, main cause of Vit D deficiency is the failure of:

The key is D. 1 alpha hydroxylation of Vit D. [There are 3 steps in the synthesis of vitamin D. i) Cholecalciferol in the skin from 7-dehydrocholesterol under the action of ultraviolet light. ii) Hydroxylation in the liver of cholecalciferol to 25-hydroxycholecalciferol. iii) Hydroxylation in the kidneys (1 alpha hydroxylation by enzyme 1-alphahydroxylase) of 25-hydroxycholecalciferol to calcitriol (1,25dihydroxyvitamin D3)].