This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Excretory System (Kidney, Bladder) Acid-Base Balance, Subject name: Physiology
1,25-dihydroxycholecalciferol is secreted by: 
Select and explain. 
A. Adrenal coex, B. Liver, C. Proximal tubules of kidney, D. Collecting ducts of kidney
C: Proximal tubules of kidney
Answer: C: Proximal tubules of kidney 
Explanation: It is the most active form of vitamin D(1,25-dihydroxycholecalciferol) / (calcitriol) When UV rays fall on the skin, 7-dehydrocholesterol in the skin is conveed into cholecalciferol. It goes to the liver, a hydroxyl group is attached to it, 25-(OH) cholecalciferol is formed. It then travels to kidneys. In the cells of the PCT, one more hydroxyl group is attached, to form the active metabolite 1,25-dihydroxycholecalciferol. Vit. D is considered a steroid hormone. It acts on the intestines, kidneys, and bone. Its chief function is to increase Ca2+ and PO42- in the ECF.