A 68-year-old man with a 3-year history of progressive memory loss and cognitive decline is brought to the psychiatric outpatient clinic by his daughter. He has difficulty with recent memory, gets lost in familiar places, and shows poor judgment. His wife reports gradual onset over months. On examination, he is alert and oriented to person and place but not to date. Which investigation is most appropriate to differentiate this presentation from delirium and confirm the underlying neurodegenerative process?