A 68-year-old man with a 3-year history of progressive memory loss and cognitive decline is brought to the psychiatry outpatient clinic. His wife reports gradual worsening of orientation and language difficulties. On examination, he is alert and oriented to person only, with a Mini-Cog score of 2/3. His vital signs are stable, and physical examination is unremarkable. Which single feature best distinguishes his underlying condition from acute delirium?