Which major symptom differentiates delirium from major neurocognitive disorders like Alzheimer's disease?

Rationale: A. Delirium typically impacts attention, awareness, and short-term memory, rather than long-term memory. Long-term memory impairment is more characteristic of Alzheimer’s disease and other major neurocognitive disorders. B. This is a hallmark feature of delirium and a key differentiating factor from major neurocognitive disorders. Patients with delirium often experience periods of lucidity alternating with confusion, with symptoms that worsen in the evening or at night (a phenomenon known as “sundowning”). This acute, fluctuating course contrasts with the gradual and steady decline seen in Alzheimer’s disease. C. Delirium typically has an acute onset over hours to days, not a gradual onset. Gradual onset is more typical of Alzheimer’s and other neurocognitive disorders. D. Delirium is usually reversible if the underlying cause is identified and treated. Major neurocognitive disorders, in contrast, are progressive and largely irreversible.