**Core Concept**
Delirium is a complex neuropsychiatric syndrome characterized by an acute onset of fluctuating cognitive impairment, often accompanied by altered levels of consciousness and changes in emotional status. This condition can result from various medical, neurological, or psychiatric disorders, and its pathophysiology involves disruptions in the normal functioning of neurotransmitter systems in the brain.

**Why the Correct Answer is Right**
Delirium is typically marked by a disturbance in consciousness, manifesting as an altered level of awareness or a change in the sleep-wake cycle. This disturbance is often accompanied by a change in cognition, such as disorientation, memory impairment, or difficulty with language and problem-solving. The pathophysiology of delirium involves the release of various neurotransmitters, including glutamate, dopamine, and acetylcholine, which can lead to an imbalance in brain activity and contribute to the development of delirium symptoms.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while it is true that delirium can be caused by various factors, including infection, medication, and substance withdrawal, it does not specifically describe a characteristic feature of delirium.
**Option B:** This option is incorrect because although delirium can be associated with hallucinations, it is not a defining feature of the condition. Hallucinations can occur in a variety of psychiatric and neurological disorders, including schizophrenia and Parkinson's disease.
**Option C:** This option is incorrect because while agitation and aggression can be symptoms of delirium, they are not the primary features of the condition. Other symptoms, such as disorientation and memory impairment, are more characteristic of delirium.

**Clinical Pearl / High-Yield Fact**
One important clinical pearl to remember is that delirium often presents with a fluctuating course, meaning that symptoms can wax and wane throughout the day. This fluctuation is a key feature that differentiates delirium from other cognitive disorders, such as dementia.

**Correct Answer:** D. Altered level of consciousness.