A 30-year-old female patient has complained of weakness and fatigability over the past 6 months. She has a 3-month acute history of severe hypertension that has not responded to antihypertensive medications. Fasting serum glucose concentration is 140 mg/dl. A CT scan of the abdomen shows a 6-cm mass in the adrenal gland affecting the secretory cells of the adrenal medulla. Which of the following structures is most likely releasing products into the bloodstream to produce the hypertension and other signs?

The mass leads to increased stimulation and secretions of the chromaffin cells of the adrenal medulla. These cells are modified postganglionic sympathetic neurons of neural crest origin, and the epinephrine (adrenaline) and norepinephrine (noradrenaline) released by these cells passes into the suprarenal (adrenal) veins. The adrenal medulla receives stimulation from preganglionic sympathetic fibers carried by the thoracic splanchnic nerves. Parasympathetic neurons are not found in the adrenal medulla and would have no participation in the effects of the tumor. In addition, the pelvic splanchnic nerves are parasympathetic and do not travel to the adrenal medulla.