Pierre has been an alcoholic for several decades. Now in his 50s, his intellectual abilities are intact, but he suffers from memory deficits and hallucinations. Based on this information, you suspect that Pierre may be afflicted with:

Pierre most likely has Korsakoff syndrome.

Korsakoff syndrom is a neurological disease caused by excessive alcohol consumption. This is because too much alcohol can deplete thiamine (vitamin B1). This thiamine defficiency then causes amnesia, hallucinations, and other neurological symptoms.