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: Physiology
Which hormone is released when serum calcium decreases: 
Select and explain. 
A. Parathormone, B. Calcitonin, C. Thyroxine, D. Adrenaline
A: Parathormone
Answer: A: Parathormone 
Explanation: Parathyroid hormone (PTH) is secreted by the parathyroid glands. Its main action is to mobilize calcium from bone and increase urinary phosphate excretion.