A 68-year-old man with a 3-year history of progressive memory loss and cognitive decline is brought to the hospital by his family. He has been increasingly forgetful, disoriented to time, and unable to perform activities of daily living. His mental status has been relatively stable over the past 6 months. Vital signs are normal, and physical examination reveals no focal neurological deficits. Which investigation is most appropriate to confirm the underlying diagnosis and exclude reversible causes?