Delirium features:

• A. A disturbance of attention and awareness with rapid onset, fluctuations, and cognitive disturbances, due to a medical condition or substance.
• B. A mood disorder characterized by persistent sadness.
• C. A sleep disorder with irregular cycles.
• D. A progressive neurodegenerative disease.

Answer: (A)

Delirium is defined by an abrupt disturbance in attention and awareness that develops over hours to days and tends to fluctuate in severity throughout the day, along with a change in cognition (such as memory deficits, disorientation, language, or perceptual disturbances). Importantly, this syndrome is caused by another medical condition or by a substance (or withdrawal), rather than being a primary psychiatric disorder or a purely neurodegenerative process. This combination of impaired attention and awareness, acute and fluctuating onset, and an underlying medical or substance-related cause is what distinguishes delirium from other conditions. The other options describe a mood disorder, a sleep-wake disorder, and a progressive neurodegenerative disease, respectively, which do not capture the core delirium features.