**Core Concept**
Glycosaminoglycans (GAGs) are a class of polysaccharides composed of repeating disaccharide units, which include a uronic acid residue, a hexosamine, and various modifications such as sulfate or iduronic acid groups. Uronic acid is a key component of GAGs, and its structure is critical for their function.

**Why the Correct Answer is Right**
Heparin is an acidic polysaccharide composed of alternating units of D-glucosamine and L-iduronic acid. However, heparan sulfate is structurally distinct from other GAGs in that it contains a higher proportion of N-sulfated glucosamine and a lower proportion of iduronic acid. Specifically, heparan sulfate contains acetyl groups instead of uronic acid, making it the GAG that does not contain uronic acid.

**Why Each Wrong Option is Incorrect**
* **Option A:** Chondroitin sulfate contains a repeating disaccharide unit of glucuronic acid and N-acetylgalactosamine, making it a uronic acid-containing GAG.
* **Option B:** Dermatan sulfate contains a repeating disaccharide unit of iduronic acid and N-acetylgalactosamine, making it a uronic acid-containing GAG.
* **Option D:** Keratan sulfate contains a repeating disaccharide unit of galactose and N-acetylglucosamine, but it is not typically classified as a GAG that contains uronic acid.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that GAGs play critical roles in cell signaling, cell adhesion, and the regulation of various physiological processes, including inflammation, angiogenesis, and tissue repair.

**Correct Answer:** C. Heparan sulfate.