A 77-year-old man with a past medical history of hypertension and osteoarthritis presents with a 6-month history of forgetfulness, difficulty finding words, and problems managing his finances. His family reports he has also become more apathetic and his personality has changed slightly. A cognitive assessment reveals impairment in multiple domains. An MRI brain shows moderate generalised atrophy and some periventricular white matter changes. Which single diagnosis is most likely?