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
[{'content': 'The blood levels of 1,25 dihydroxycholecalciferol is positively regulated by:- \nSelect and explain. \nA. PTH, B. Calcium, C. Magnesium, D. 25 OH cholecalciferol', 'role': 'user'}, {'content': 'Answer: A \nExplanation: PTH - PTH is necessary for conversion of 25-hydroxy chole calciferol to 1,25 - dihydroxy cholecalciferol: Calcium: Plasma concentration of 1,25 ( OH)2 D3 is inversely affected by Ca2+ concentration in plasma. The rate of PTH secretion is greatly suppressed when plasma Ca2+ concentration rise above 9-10 mg/100gm If Ca2+ concentration below this level - | es conversion of 25 hydroxycholecalciferol to 1,25 dihydroxycholecalciferol in the kidney At higher calcium concentration when PTH is suppressed the 25 Hydroxycholecalciferol is conveed to a different component - 24, 25- Dihydroxy cholecalciferol & that has almost no vitamin D defect', 'role': 'assistant'}]