<prompt>  A 35-year-old man comes to the physician because of a lump in his neck. Examination shows a thyroid nodule, Laboratory studies show serum calcitonin is elevated, and serum calcium is normal Despite high serum calcitonin levels, which of the following mechanisms best explains the normal calcium levels in this patient?Concurrent parathyroid adenomaConcurrent parathyroid hyperplasiaHigh levels of calcitonin down reQulates its receptorIncrease in PTH in response to hypocalcemia 