**Core Concept**
Lewy body dementia (LBD) is a neurodegenerative disorder characterized by the presence of Lewy bodies, abnormal protein clumps that disrupt cognitive and motor functions. The pathophysiology of LBD involves a combination of cholinergic dysfunction, dopamine system degeneration, and neurotransmitter imbalance.

**Why the Correct Answer is Right**
Lewy body dementia is a progressive disorder that typically affects individuals over the age of 65. It is the second most common form of dementia after Alzheimer's disease, accounting for approximately 10-20% of dementia cases. The clinical presentation of LBD often includes fluctuations in cognitive function, visual hallucinations, and parkinsonian motor symptoms. The disease is also associated with REM sleep behavior disorder and a heightened risk of falls due to postural instability.

**Why Each Wrong Option is Incorrect**
**Option A:** This statement is incorrect because Lewy body dementia is indeed associated with fluctuations in cognitive function, but it is not the only dementia subtype to exhibit such fluctuations. However, the characteristic fluctuations in LBD are a hallmark feature that distinguishes it from other dementias.
**Option B:** This statement is incorrect because visual hallucinations are a common and characteristic feature of Lewy body dementia, often appearing in the early stages of the disease. These hallucinations can be vivid and detailed, and may involve animals, people, or objects.
**Option C:** This statement is incorrect because Lewy body dementia does not typically present with a gradual decline in cognitive function, as seen in Alzheimer's disease. Instead, LBD is characterized by a more rapid progression of cognitive decline, often with a fluctuating course.

**Clinical Pearl / High-Yield Fact**
A key feature that distinguishes Lewy body dementia from other dementias is the presence of REM sleep behavior disorder, which is often the earliest manifestation of the disease. This disorder is characterized by acting out dreams during sleep, often resulting in violent or aggressive behavior.

**Correct Answer: A. This statement is incorrect because Lewy body dementia is indeed associated with fluctuations in cognitive function, but it is not the only dementia subtype to exhibit such fluctuations.