This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Nutrition and Metabolic disorders,Systemic conditions ,Genodermatoses, Connective tissue disorders,Photodermatoses, Subject name: Dental
Vitamin D is synthesized by? 
Select and explain. 
A. Keratinocytes, B. Granular cells, C. Melanocytes, D. Prickle cells
A: Keratinocytes
Answer: A: Keratinocytes 
Explanation: 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 1,25(OH)2D they produce.