Which laboratory finding supports the diagnosis of pheochromocytoma?

• A. Decreased urinary catecholamines
• B. Hyperkalemia
• C. Increased 24-hour urinary metanephrines and catecholamines
• D. Low metanephrine levels

Answer: (C)

Pheochromocytoma causes overproduction of catecholamines from a tumor in the adrenal medulla, so the most reliable laboratory clue is increased metabolites and catecholamines in the urine. Metanephrines are produced when catecholamines are metabolized, and they tend to reflect ongoing tumor activity more consistently than episodic catecholamine surges. Measuring them over a 24-hour period provides a sensitive screen for the tumor. When 24-hour urinary metanephrines and catecholamines are elevated, it strongly supports the diagnosis and prompts further imaging to localize the tumor.

In contrast, decreased urinary catecholamines would not fit this picture, hyperkalemia is not characteristic of pheochromocytoma, and low metanephrine levels would argue against the diagnosis.