Correct!

The answer is: An amino sugar

From the article Ingredient Breakdown and Key Terminology:
What is Glucosamine?

Glucosamine is an amino sugar produced in the body that provides a key building block for the synthesis of GAGs and proteoglycans. Glucosamine is found in the tissues of tendons, ligaments, cartilage, synovial fluid, skin, heart valves and the eye. Glucosamine has been shown to stimulate GAG synthesis in the cartilage matrix and synovial fluid. The body uses glucosamine to produce the GAGs, which then give shape, elasticity and rigidity to the aforementioned connective tissues as well as intervertebral discs, the interstitium and mucous membranes. A high level of GAGs enables the joints to hold more water thereby increasing flexibility, cushioning ability and resiliency. Glucosamine makes up 50 percent of hyaluronic acid- the principal GAG found in synovial fluid.

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