**Core Concept**
Dementia is a chronic and persistent disorder of the mental processes caused by brain disease or injury, leading to impaired memory, thinking, and behavior. It is characterized by a decline in cognitive function, including memory loss, communication difficulties, problem-solving deficits, and mood changes.

**Why the Correct Answer is Right**
Dementia is a clinical syndrome that involves multiple cognitive domains, including memory, language, executive function, and visuospatial skills. The core features of dementia include a noticeable and persistent decline in cognitive function, with significant impairment in daily life activities. The diagnosis of dementia is based on a comprehensive clinical evaluation, including medical history, physical examination, laboratory tests, and neuropsychological assessments.

**Why Each Wrong Option is Incorrect**
**Option A:** This is a feature of dementia.
**Option B:** This is also a feature of dementia.
**Option C:** This is actually a characteristic of depression, not dementia.

**Clinical Pearl / High-Yield Fact**
It's essential to differentiate between dementia and depression, as they can present with similar symptoms, such as memory loss and mood changes. However, depression is often reversible with treatment, whereas dementia is a chronic and progressive condition.

**Correct Answer: C. Depression.**