A 24-year-old individual is brought to a clinic by their family. The family reports that for the past ten days, the individual has been sleeping only two hours a night but has boundless energy, has been speaking in a rapid, pressured manner that is hard to follow, and impulsively purchased a non-refundable, first-class ticket for a trip around the world, despite having significant debt. The individual reports feeling 'invincible' and 'on top of the world.' When asked about their mood history, the individual denies ever having experienced a significant period of depression. Based on this presentation, what is the most likely diagnosis, and what is the single, essential feature described that confirms it?