<prompt> Vitamin D is synthesized by?KeratinocytesGranular cellsMelanocytesPrickle cells  <response>: The keratinocytes of the skin are unique in being not only the primary source of vitamin D for the body, but in possessing the enzymatic machinery to metabolize vitamin D to its active metabolite 1,25 dihydroxy vitamin D. Fuhermore, these cells also express the vitamin D receptor VDR that enables them to respond to the 1,25OH2D they produce. 