**Question:** "Egg-shell calcifications" are seen in

A. Dental calculus
B. Calcium deposits in the liver
C. Senile plaques in Alzheimer's disease
D. Kidney stones

**Correct Answer:** C. Senile plaques in Alzheimer's disease

**Core Concept:**
Egg-shell calcifications refer to the appearance of calcium deposits in a tissue or organ, where the calcifications appear as thin, translucent layers surrounding a central area of lower density. This imaging feature can be observed in various conditions involving calcium deposition.

**Why the Correct Answer is Right:**
Egg-shell calcifications are commonly found in senile plaques, which are a hallmark feature of Alzheimer's disease. These plaques are extracellular aggregates of amyloid-beta protein, which undergo mineralization, forming calcium deposits. The thin, translucent layers represent the calcium deposition, while the lower density in the center represents the amyloid core.

**Why Each Wrong Option is Incorrect:**

A. Dental calculus (also known as tartar) is a hard, yellowish-white deposit on the tooth surface, primarily composed of calcium and phosphate salts. While it can have a similar appearance to egg-shell calcifications, it does not involve Alzheimer-like plaques or Alzheimer's disease.

B. Calcium deposits in the liver (hepatolithiasis) and kidney stones (urolithiasis) are distinct entities with different causes and appearances. These calcifications are not associated with Alzheimer's disease or egg-shell calcifications.

D. Kidney stones are mineral deposits, primarily composed of calcium oxalate or calcium phosphate, which form in the urinary tract. Egg-shell calcifications are not observed in kidney stones.

**Clinical Pearl:**
Egg-shell calcifications in the context of Alzheimer's disease represent the mineralization of senile plaques, which is a characteristic feature of this neurodegenerative disorder affecting the central nervous system. Understanding this core concept helps in differentiating it from other conditions involving calcium deposition and distinguishing it from the more common causes of egg-shell calcifications.