t,eociN eht etsm ayss pssrro"eocr ni het snik"
D3 loecc(eioarlf)hcl mrof oexupres of knis rtautsm( sab)lea ot nus, itsnioeng of f,sih iml,k .latpns
D2 feoco)(lglirearc ofmr gtesionni fo p,sntla fngiu, .eyssat
ohBt dneetvorc ot -O25H D3 r(aesgot rm)of ni elriv and to het veitca mfro 21,)(5-H2O D3 o)(tliirccla in indeky.
The key bit in this question that makes the answer Cholecalciferol rather than 7-dehydrocholesterol is the nugget that the patient "rigorously avoids exposure to the sun." Conversion of 7-dehydrocholesterol to Cholecalciferol can only occur when the skin is exposed to sunlight. Therefore, decreased production of Cholecalciferol (D3) is the better answer.
This picture always helps me remember Vitamin D metabolism.