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: Medicine
[{'content': 'A 35-year-old man comes to the physician because of a lump in his neck. Examination shows \na thyroid nodule, Laboratory studies show serum calcitonin is elevated, and serum calcium is \nnormal \nDespite high serum calcitonin levels, which of the following mechanisms best explains the normal calcium levels in this patient? \nSelect and explain. \nA. Concurrent parathyroid adenoma, B. Concurrent parathyroid hyperplasia, C. High levels of calcitonin down reQulates its receptor, D. Increase in PTH in response to hypocalcemia', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]