A 41-year-old female presents for evaluation of recurrent episodes of intense anxiety, palpitations, diaphoresis, tremor, and a sense of impending doom occurring 3-4 times per week over the past 4 months. She describes each episode as sudden in onset, lasting 15-30 minutes, and occurring both at rest and with activity without identifiable psychological triggers. Between episodes, she feels well. Her primary care provider diagnosed panic disorder and initiated sertraline 6 weeks ago with no improvement. She reports recent weight loss of 12 pounds despite normal appetite. Her blood pressure during an observed episode in the office is 210/120 mmHg with a heart rate of 128 bpm, and between episodes her blood pressure is 148/92 mmHg. She appears pale and diaphoretic during the episode. Plasma free metanephrines are markedly elevated. Which of the following is the most likely diagnosis?