This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Adrenal Gland, Subject name: Pathology
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? 
Select and explain. 
A. Serum C-peptide, B. Serum calcitonin, C. Urinary vanillylmandelic acid, D. Urinary aldosterone
C: Urinary vanillylmandelic acid
Answer: C: Urinary vanillylmandelic acid 
Explanation: - 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.