## **Core Concept**
Alzheimer's disease is a progressive neurodegenerative disorder characterized by cognitive decline and memory loss. The pathognomonic feature of Alzheimer's disease is the presence of **neurofibrillary tangles** and **amyloid plaques** in the brain. These histopathological changes are hallmarks of the disease.

## **Why the Correct Answer is Right**
The correct answer, neurofibrillary tangles, are a key feature of Alzheimer's disease. These tangles are composed of **hyperphosphorylated tau protein** that accumulates inside neurons, leading to their dysfunction and death. The presence of neurofibrillary tangles, along with amyloid plaques, is a diagnostic criterion for Alzheimer's disease.

## **Why Each Wrong Option is Incorrect**
* **Option A:** While amyloid plaques are also a hallmark of Alzheimer's disease, they are not as specific to the disease as neurofibrillary tangles. Amyloid plaques can be found in other conditions, such as cerebral amyloid angiopathy.
* **Option B:** Lewy bodies are characteristic of **Lewy body dementia**, a different neurodegenerative disorder. They are not a feature of Alzheimer's disease.
* **Option C:** Cerebellar atrophy is a feature of various neurodegenerative diseases, including **multiple system atrophy** and **spinocerebellar ataxia**. It is not specific to Alzheimer's disease.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that Alzheimer's disease is the most common cause of **dementia** in older adults. The presence of neurofibrillary tangles and amyloid plaques in the brain is diagnostic of Alzheimer's disease. Clinicians should consider Alzheimer's disease in patients with progressive cognitive decline and memory loss.

## **Correct Answer:** D. Neurofibrillary tangles.