<prompt> The most reliable test in the diagnosis of pheochromocytoma is24 hour urinary metanephrineUrinary catecholaminesUrinary catecholaminesBasal plasma catecholamines  <response>: Pheochromocytomas are neoplasms of chromaffin cells,which synthesize and release catecholamines. The lab diagnosis of this condition is based on demonstration of increased urinary excretion of free catecholamines and their metabolites; vanilylmandelic acid and metanephrines. ref:ROBBINS Text book of pathology 