PAED-0399

A 13-year-old girl presents with episodes of feeling her heart racing and pounding, dizziness, and sweating. Her mother reports that she appears slightly pale and feels frightened during these episodes. These episodes occur at different times during the day and a few times a week. There is no clear history suggestive of a trigger. She has no other known medical problems. Which of the following is the most probable underlying cause for these episodes?