A 72-year-old woman with a 5-year history of insidious memory loss, word-finding difficulty, and progressive functional decline is brought by her son to the psychiatry outpatient clinic. She is oriented to person and place but not to date. She is calm, cooperative, and denies hallucinations or mood symptoms. Cognitive testing shows MMSE 18/30. Her son asks what investigations are needed to confirm the diagnosis and guide treatment. What is the most appropriate next step?