This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Endocrine System, Subject name: Physiology
PTH does what directly? 
Select and explain. 
A. Controls the rate of 25-hydroxycholicalciferol formation, B. Controls the rate of calcium transpo in the mucosa of the small intestine, C. Controls the rate of formation of calcium-binding protein, D. Controls the rate of formation of 1,25-dihydroxycholicalciferol
D: Controls the rate of formation of 1,25-dihydroxycholicalciferol
Answer: D: Controls the rate of formation of 1,25-dihydroxycholicalciferol 
Explanation: Parathyroid hormone acts in the renal coex to stimulate the reaction forming 1,25-dihydroxycholicalciferol from 25-hydroxycholicalciferol. It has no effects on other the other reactions