A 26-year-old man is brought to hospital after a motorcycle accident with a brief loss of consciousness. Once he is alert again, he can lay down new memories normally, but he cannot recall anything from the twenty minutes immediately before the crash, and over the following days that gap gradually shrinks to only the last minute or two before impact. What is this?

This is retrograde amnesia: loss of memory for a period preceding the onset of the causal event, here the head injury. The shrinking gap over the following days is typical of the acute post-traumatic form. It is not anterograde amnesia, which is the inability to lay down new memories after the event — the question states this ability is intact. It is not confabulation, since there is no fabricated material filling the gap, just an acknowledged blank. And it is not Korsakoff's syndrome, which is a chronic amnesic syndrome from thiamine deficiency, not a single traumatic event.