A 78-year-old man presents with a 6-month history of progressive memory loss, difficulty with word-finding, and getting lost in familiar places. His wife reports he has also become more apathetic. A Mini-Mental State Examination (MMSE) score is 22/30. Blood tests including FBC, U&Es, LFTs, TFTs, B12, and folate are normal. What is the most appropriate next step in his diagnostic work-up according to NICE NG97?