**Core Concept**
Delirium is a complex neuropsychiatric disorder characterized by an acute onset of fluctuating cognitive impairment, often accompanied by altered consciousness, disorganized thinking, and perceptual disturbances. It is a medical emergency that requires prompt recognition and management to prevent complications and improve outcomes.

**Why the Correct Answer is Right**
Delirium is typically associated with a rapid onset and fluctuating course, which distinguishes it from other cognitive disorders like dementia. The underlying pathophysiology involves disruptions in normal sleep-wake cycles, neurotransmitter imbalances (e.g., acetylcholine, dopamine), and inflammation. These factors contribute to the characteristic features of delirium, including confusion, disorientation, and impaired attention and memory.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because delirium is not typically associated with a gradual onset, which is more characteristic of dementia.
**Option B:** This option is incorrect because while anxiety and agitation may be present in delirium, they are not defining features of the disorder.
**Option C:** This option is incorrect because while delirium can be caused by medication side effects, this is not a defining feature of the disorder.

**Clinical Pearl / High-Yield Fact**
A key feature of delirium is its fluctuating course, which can make diagnosis challenging. Clinicians should be aware of the importance of monitoring patients for changes in mental status and cognitive function, particularly in the setting of medical illness or medication changes.

**Correct Answer: D. Acute onset and fluctuating course.**