<prompt> A 26-year-old woman has episodic hypeension with headache, diaphoresis, and palpitation. Which of the following diagnostic procedures would be most useful in evaluating the possibility that pheochromocytoma might be the cause of these findings?Serum C-peptideSerum calcitoninUrinary vanillylmandelic acidUrinary aldosterone  <response>: - Urinary vanillylmandelic acid metanephrines - metabolites of epinephrine norepinephrine - markedly elevated in pheochromocytoma. - Serum C-peptide is elevated in insulinoma. - Serum calcitonin is sometimes used to screen for medullary carcinoma of the thyroid. 