Thyroid Disorders USMLE Step 1 Practice Question

A 52-year-old man presents to the emergency department with a 2-week history of profuse watery diarrhea (8-10 bowel movements daily) and recurrent episodes of facial flushing and diaphoresis. He reports no abdominal pain, fever, or recent antibiotic use. Past medical history is significant for total thyroidectomy and modified neck dissection 3 years ago for medullary thyroid carcinoma (MTC). Vital signs: BP 132/78 mmHg, HR 98/min, RR 16/min, temperature 36.8°C. Physical examination is unremarkable. Laboratory studies show: serum calcium 9.2 mg/dL (normal 8.5-10.2), serum phosphate 3.5 mg/dL (normal 2.5-4.5), plasma calcitonin 520 pg/mL (normal <10), and 24-hour urinary metanephrines are normal. Which of the following mediators is most likely responsible for this patient's presenting symptoms?