## **Core Concept**
Pigment stones are a type of gallstone primarily composed of bilirubin and calcium salts. They are distinct from cholesterol stones, which are mainly composed of cholesterol. The classification and composition of gallstones are crucial for understanding their formation and treatment.

## **Why the Correct Answer is Right**
Pigment stones are defined by their composition, which includes a significant amount of bilirubin and calcium salts, with minimal cholesterol content. By definition, pigment stones contain less than 30% cholesterol. This characteristic differentiates them from cholesterol stones, which are predominantly made of cholesterol (> 70%). Therefore, the correct answer indicating less than 30% cholesterol content is accurate for pigment stones.

## **Why Each Wrong Option is Incorrect**
- **Option A:** More than 70% - This is incorrect because it describes the composition of cholesterol stones, not pigment stones.
- **Option B:** 30-50% - This range is also incorrect for pigment stones as it might overlap with mixed stones but does not accurately represent the definition of pigment stones.
- **Option D:** 50-70% - This option is incorrect because it suggests a higher cholesterol content, which is not characteristic of pigment stones.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that pigment stones are often associated with hemolytic diseases (e.g., sickle cell disease), infections of the biliary tract, or other conditions leading to increased bilirubin production. They are usually smaller, darker, and more numerous than cholesterol stones.

## **Correct Answer:** C. <30%.