MSFinals-5476

A 56-year-old man is brought to the emergency department by ambulance, after being found confused by members of the public for the fifth time in the past month. Upon review, he tells you that he came here by bicycle after spending the afternoon with his friends doing shopping, and then later tells you he spent today in the pub with his new dog. He smells strongly of alcohol and you notice a near-empty bottle of unlabelled spirit with him.
On examination, he has an ataxic gait, dysdiadochokinesia and horizontal nystagmus.
When you go back later to see him, he has forgotten your previous interaction.
Which of the following explains his signs and symptoms?