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
[{'content': 'Which hormone is released when serum calcium decreases: \nSelect and explain. \nA. Parathormone, B. Calcitonin, C. Thyroxine, D. Adrenaline', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Parathyroid hormone (PTH) is secreted by the parathyroid glands. Its main action is to mobilize calcium from bone and increase urinary phosphate excretion.', 'role': 'assistant'}]