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': 'What is the physiological role of calcitonin in metabolism? \nSelect and explain. \nA. Lowers Ca2+ levels, B. Increases Ca2+ levels, C. Increases PO4 levels, D. None of the above', 'role': 'user'}, {'content': "Answer: A \nExplanation: Calcitonin lowers circulating calcium and phosphate levels. It exes its calcium-lowering effect by inhibiting bone resorption. This action is direct, and calcitonin inhibits the activity of osteoclasts in vitro. It also increases Ca2+ excretion in the urine. Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 21. Hormonal Control of Calcium & Phosphate Metabolism & the Physiology of Bone. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds), Ganong's Review of Medical Physiology, 24e.", 'role': 'assistant'}]