A 79-year-old woman is brought by a long-term care (LTC) facility staff member to your...

Delirium is the most likely diagnosis because the change is acute (3 days) with fluctuating level of activity (quiet at times, agitated at others) and prominent inattention (does not finish answers, “drifts off” during conversation). MCCQE objectives emphasize distinguishing delirium from dementia: dementia is typically chronic and progressive , whereas delirium is a sudden, reversible syndrome characterized by disturbed attention and awareness, often with altered sleep–wake cycle, perceptual disturbances, and psychomotor changes (hyperactive, hypoactive, or mixed). This patient’s new aggression, confusion, and variable behavior strongly suggest delirium superimposed on existing vascular dementia—a common and high-risk scenario in LTC residents. Major depressive disorder can cause apathy and reduced concentration but does not typically cause a fluctuating course with acute confusion. Lewy body dementia is associated with chronic cognitive fluctuations and visual hallucinations/parkinsonism, not a sudden 3-day deterioration. Late-onset delusional disorder would feature fixed delusions with otherwise clearer consciousness. The next clinical priority is to search for and treat underlying causes (infection, medications, metabolic disturbance, pain, dehydration) while ensuring safety and supportive care.