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The most appropriate initial screening test for pheochromocytoma is checking plasma free metanephrines. Pheochromocytoma is a tumor that secretes catecholamines, which can lead to symptoms such as hypertension, palpitations, sweating, and anxiety. The metabolites of catecholamines, specifically metanephrines (metanephrine and normetanephrine), are released into the bloodstream and are excellent markers for the presence of this tumor.

Plasma free metanephrines are typically measured because they have a higher sensitivity for detecting pheochromocytoma compared to other tests. If elevated levels are found, further confirmatory testing can be performed, such as a 24-hour urine collection for catecholamines or metanephrines, or imaging studies to locate the tumor.

The other tests listed are not appropriate for initial screening. For example, measuring 24-hour urine cortisol levels evaluates adrenal cortisol production and is not directly related to pheochromocytoma. The dexamethasone suppression test is used primarily to differentiate Cushing's syndrome and does not apply to pheochromocytoma. Finally, measuring serum aldosterone levels is more relevant for conditions like primary hyperaldosteronism and does not screen for