Review Question - QID 110011

A 57-year-old male presents to his primary care physician with upper abdominal pain. He reports a 3-month history of mild epigastric pain that improves with meals. He has lost 15 pounds since his symptoms started. His past medical history is notable for gynecomastia in the setting of a prolactinoma for which he underwent surgical resection over 10 years prior. He has a 15-pack-year smoking history, a history of heroin abuse, and is on methadone. His family history is notable for parathyroid adenoma in his father. His temperature is 98.8°F (37.1°C), blood pressure is 125/80 mmHg, pulse is 78/min, and respirations are 18/min. This patient’s symptoms are most likely due to elevations in a substance with which of the following functions?