uk-dgm MCQ (SBA): Apathy vs Depression in Alzheimer's Disease

A 83-year-old woman with a CFS of 5 and moderate Alzheimer's disease is seen by her GP. Her daughter reports that she has been increasingly apathetic over the past 3 months - sitting in her chair all day, not engaging in conversation, and showing no interest in her surroundings. She does not appear sad or tearful. She has no sleep disturbance. Her MMSE has been stable at 15/30. What is the MOST likely diagnosis?