**Core Concept**
Hyaluronic acid (HA) is a naturally occurring glycosaminoglycan (GAG) found in connective tissue, playing a crucial role in maintaining tissue hydration, elasticity, and structure. Its composition and structure are essential for its biological functions.

**Why the Correct Answer is Right**
Hyaluronic acid is composed of a disaccharide repeat unit consisting of glucuronic acid and N-acetylglucosamine. The repeating disaccharide units are linked together through glycosidic bonds, forming a long polysaccharide chain. The molecular weight of HA can vary greatly, ranging from 500 to 20 million Da, depending on the tissue source and biological function. The high molecular weight of HA allows it to form a gel-like matrix, which is essential for maintaining tissue hydration and structure.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because hyaluronic acid is not composed of glucose and fructose molecules.
**Option B:** This option is incorrect because hyaluronic acid does not contain a sulfate group, unlike other glycosaminoglycans such as heparin.
**Option C:** This option is incorrect because hyaluronic acid is not composed of a single disaccharide unit, but rather a repeating disaccharide unit.

**Clinical Pearl / High-Yield Fact**
Hyaluronic acid is often used in cosmetic and medical applications, including dermal fillers and wound healing products, due to its ability to retain large amounts of water and promote tissue hydration.

**Correct Answer:** D. A disaccharide repeat unit consisting of glucuronic acid and N-acetylglucosamine.