A 20-year-old man suffered a significant closed head injury after a diving accident. He is left with minor memory impairment but makes a full recovery. A few months later, he has a witnessed loss of consciousness with some arm and leg twitching. After the event, he is disoriented for 2 hours and then gradually returns to baseline. There was no incontinence or tongue biting, and his examination is normal. Which of the following is the most likely diagnosis?