Review Question - QID 213484

A 41-year-old man presents to the emergency department with a 1-month history of increased difficulty when swallowing solids. He also says that he has been experiencing weight loss and muscle spasms. He has no significant past medical history but his family has several members that had rare tumors such as pheochromocytomas. On physical exam, he is found to have an enlarged thyroid gland and a biopsy is obtained from this mass. Histology of the removed section was stained and the result is shown in Figure A. The cells from which this mass is derived normally produce a substance with which of the following functions?