A 25 year old male repos episodic "spells" characterized by palpitations, sweating, nervousness, and feelings of anxiety. On examination, the man's blood pressure is 165/95 mm Hg. Plasma norepinephrine is 450 pg/mL (normal, 150-400 pg/mL), plasma epinephrine is 115 pg/mL (normal, 25-100 pg/mL), and 24-hour urinary VMA is 11 mg (normal, < 8 mg). Which of the following is the most likely cause of the patient's hypeension?