**Core Concept**
Fibrous dysplasia is a benign bone disorder characterized by the replacement of normal bone tissue with fibrous connective tissue, leading to the formation of abnormal bone structures. This condition affects the bone's ability to absorb and distribute loads, resulting in various clinical manifestations.

**Why the Correct Answer is Right**
In fibrous dysplasia, the affected bone exhibits characteristic X-ray findings, including ground-glass appearance, cortical thinning, and periosteal reaction. The ground-glass appearance is due to the replacement of normal bone with fibrous tissue, which scatters X-rays, giving a characteristic "salt-and-pepper" appearance. The cortical thinning is a result of the resorption of normal bone by the fibrous tissue. The periosteal reaction is a response to the abnormal bone formation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because fibrous dysplasia does not typically exhibit a "bone-within-a-bone" appearance, which is characteristic of other bone disorders.

**Option B:** This option is incorrect because fibrous dysplasia does not typically exhibit a "cystic" appearance, which is characteristic of other bone disorders.

**Option C:** This option is incorrect because fibrous dysplasia does not typically exhibit a "sclerotic" appearance, which is characteristic of other bone disorders.

**Clinical Pearl / High-Yield Fact**
A key radiological feature of fibrous dysplasia is the presence of a "ground-glass" appearance on X-ray, which is due to the replacement of normal bone with fibrous tissue.

**Correct Answer:** C. Cortical thinning with ground-glass appearance.