Memento

A 60 year old man with an insignificant past medical history presents with sudden onset of confusion. His wife witnessed him and said he was disoriented to time and repetitively asked questions about the date and the place, sounding almost like a broken record. His wife, a psychologist, tested digit-span and the patient was able to repeat a 7-digit number immediately, but delayed recall was impaired. He also had some amnesia of recent events of the last few days. Oddly enough, the patient was able to continue cooking and afterward, he played piano perfectly. He had no garbled speech, no facial droop, no weakness, and no focal or lateralizing signs. After six hours, the patient recovers completely. He can't recall what happened during those six hours. He had no other complaints. You see him in the emergency department after the entire episode has resolved, and your physical exam is perfectly normal. Labs are all normal. EKG is normal sinus rhythm.

Challenge: What's your most likely diagnosis?