**Core Concept**
Delirium and dementia are two distinct cognitive disorders that can be differentiated based on their clinical presentation, pathophysiology, and underlying causes. Delirium is a state of acute confusion or altered mental status, often precipitated by medical conditions, medications, or environmental factors, whereas dementia is a chronic, progressive decline in cognitive function.

**Why the Correct Answer is Right**
Delirium is characterized by a fluctuating course, with rapid changes in mental status, attention, and cognitive function. This is in contrast to dementia, which typically presents with a gradual decline in cognitive function over months or years. The correct answer option likely highlights this key difference in the clinical presentation of delirium and dementia.

**Why Each Wrong Option is Incorrect**
**Option A:** This option might suggest that delirium and dementia can be differentiated based on age of onset. However, both conditions can occur at any age, and age is not a reliable distinguishing feature.

**Option B:** This option might propose that delirium is caused by a specific type of infection, while dementia is caused by a different type of infection. However, both delirium and dementia can be caused by a wide range of underlying medical conditions, including infections, and cannot be differentiated based on the specific cause.

**Option C:** This option might suggest that delirium is a reversible condition, while dementia is irreversible. However, both delirium and dementia can have reversible underlying causes, and the reversibility of the condition does not reliably distinguish between the two.

**Clinical Pearl / High-Yield Fact**
A key clinical pearl is to look for a fluctuating course and rapid changes in mental status when differentiating delirium from dementia. Additionally, the Mini-Mental State Examination (MMSE) can be used to assess cognitive function in patients with suspected dementia, but it is not typically used to diagnose delirium.

**Correct Answer:** C.