**Core Concept**
Dementia is a chronic or persistent disorder of the mental processes caused by brain disease or injury and marked by memory disorders, personality changes, and impaired reasoning. It encompasses a range of cognitive, emotional, and behavioral symptoms that affect an individual's quality of life.

**Why the Correct Answer is Right**
Dementia is characterized by the progressive loss of cognitive function, including memory, language, problem-solving, and other thinking abilities. Alzheimer's disease is the most common cause of dementia, accounting for 60-80% of cases. The pathophysiology of dementia involves the accumulation of amyloid-beta plaques and tau tangles in the brain, leading to neuronal damage and death.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not specify the most common cause of dementia.
**Option B:** This option is incorrect as it does not address the pathophysiology of dementia.
**Option C:** This option is incorrect as it does not accurately describe the symptoms of dementia.

**Clinical Pearl / High-Yield Fact**
One of the key features of dementia is the presence of cognitive and functional decline, which can be measured using standardized assessment tools such as the Mini-Mental State Examination (MMSE).

**Correct Answer:** A. Alzheimer's disease is the most common cause of dementia.