This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Unknown
The most important regulator of serum 1,25 (OH)2 vitamin D concentration is: 
Select and explain. 
A. Serum calcium, B. Serum magnesium, C. Serum 25 (OH) vitamin D, D. Parathyroid hormone
D: Parathyroid hormone
Answer: D: Parathyroid hormone 
Explanation: A major function of parathyroid hormone is to act as a trophic hormone to regulate the rate of formation of 1,25(OH)2 vitamin D. The mechanism by which parathyroid hormone exerts this effect may be secondary to its effects on phos­phorus metabolism. Other hormones, including prolactin and estrogen, also may playa role in stimulating the production of 1,25(OH)2 vitamin D.