## **Core Concept**
Glycosaminoglycans (GAGs) are negatively charged polysaccharides that play critical roles in the extracellular matrix and on cell surfaces. They are composed of repeating disaccharide units and are involved in various biological processes, including cell signaling, maintaining tissue structure, and interacting with growth factors. Iduronic acid is a unique component of certain GAGs.

## **Why the Correct Answer is Right**
The correct answer, **Heparan sulfate** and **Dermatan sulfate**, both contain iduronic acid, but **Heparan sulfate** uniquely and predominantly features **iduronic acid** in its structure. Iduronic acid is specifically found in heparan sulfate and dermatan sulfate; however, heparan sulfate is more uniquely highlighted for this feature. Heparan sulfate is synthesized by various cells and plays significant roles in anticoagulation, lipid metabolism, and cell growth regulation.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Hyaluronic acid does not contain iduronic acid; instead, it is composed of glucuronic acid and N-acetylglucosamine.
- **Option B:** Chondroitin sulfate contains glucuronic acid or N-acetylgalactosamine and galactosamine but not iduronic acid.
- **Option D:** Keratan sulfate is composed of galactose and N-acetylglucosamine and does not contain iduronic acid.

## **Clinical Pearl / High-Yield Fact**
A key clinical correlation is that **heparan sulfate**'s presence of iduronic acid contributes to its anticoagulant properties when it is part of heparin, which is widely used to prevent and treat thrombosis. This highlights the importance of understanding GAG structure-function relationships.

## **Correct Answer:** . Heparan sulfate