A patient with a pheochromocytoma is secreting large amounts of norepinephrine into the bloodstream. In a normal individual, this compound is usually released from the adrenal medulla in response to which of the following?

In the normal individual, release of the catecholamine norepinephrine is under neuroendocrine control, with acetylcholine serving as the local neurotransmitter that triggers its release. This normal control is disrupted in patients with pheochromocytoma. The catecholamine epinephrine is also secreted by the adrenal medulla, but does not regulate norepinephrine secretion. Metanephrine, normetanephrine, and vanillylmandelic acid are all catecholamine degradative products that may become elevated if catecholamine production is increased by a pheochromocytoma.