**Core Concept**
Delirium and dementia are two distinct neurocognitive disorders that differ in their clinical presentation, pathophysiology, and underlying causes. Delirium is characterized by an acute onset of fluctuating cognitive impairment, often accompanied by altered consciousness, whereas dementia is a chronic, progressive condition marked by a decline in cognitive function.

**Why the Correct Answer is Right**
Delirium is distinguished from dementia by its acute onset, fluctuating course, and presence of altered consciousness. This is often due to an underlying medical condition, medication side effect, or substance intoxication. In contrast, dementia is a gradual decline in cognitive function, typically over months or years, with no clear fluctuations in symptoms. The pathophysiology of delirium involves disruptions in neurotransmitter systems, such as acetylcholine and dopamine, whereas dementia is associated with the accumulation of amyloid-beta plaques and tau protein tangles in the brain.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specifically address the key difference between delirium and dementia. While both conditions may involve cognitive impairment, the acute onset and fluctuating course of delirium are crucial distinguishing features.
**Option B:** This option is incorrect because it refers to a symptom that can be present in both delirium and dementia, but does not capture the essential difference between the two conditions. Both conditions can involve memory loss, but the context and pattern of memory impairment differ significantly between delirium and dementia.
**Option C:** This option is incorrect because it is a characteristic of both delirium and dementia, but does not provide a clear distinction between the two conditions. Both conditions can involve difficulty with concentration and attention, but the underlying causes and clinical presentation differ significantly.

**Clinical Pearl / High-Yield Fact**
To remember the key difference between delirium and dementia, use the acronym "FLIC": Fluctuating course, Lability of symptoms, Inattention, and Clouded consciousness. These features are characteristic of delirium, whereas dementia is typically marked by a gradual decline in cognitive function without significant fluctuations.

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