Huh?

A 60 year old with migraines is brought to your emergency department by his daughter for acute onset confusion. They were at the mall, and he was in his usual state of good health when suddenly he became confused. He didn't know what time it was, and kept asking about the date and where they were. "He sounded like a broken record." No matter how much he was oriented, he simply could not remember what was told to him. He still knew who he was and could do complex tasks - in fact, he was able to tie a tie. This all started an hour ago. The only other symptom is a mild headache.

When you see the patient, he is unchanged. The patient's vitals and neurologic exam are otherwise normal. He has no deficits in cranial nerves, motor, sensory, or coordination. Immediate recall - asking him to repeat back what he heard - was intact, but delayed recall was impaired. A stat head CT is normal. An MRI is shown below:

When the patient gets back from MRI, about 6 hours after the initial onset, he is back to normal. He is alert, oriented, has a normal mini mental status exam, and has a normal neurologic exam. He does not remember the last six hours, however.

Challenge: What's the diagnosis?

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