**Core Concept**
The Voisins ring is a characteristic radiographic finding associated with a specific type of bone lesion. It represents a calcified ring of bone that forms around a central nidus, typically seen in osteoid osteoma. This phenomenon is a result of the body's attempt to wall off the nidus, leading to the formation of a ring of reactive bone.

**Why the Correct Answer is Right**
The Voisins ring is a pathognomonic sign of osteoid osteoma, a benign bone tumor that typically affects the diaphysis of long bones. The ring forms as a result of the body's response to the tumor, with the inner layer of the ring being the original cortical bone and the outer layer being the new bone formed in response to the tumor. This ring is visible on plain radiographs and is a key diagnostic feature of osteoid osteoma.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the Voisins ring is not typically associated with osteoblastoma, which is a different type of benign bone tumor.
**Option B:** This option is incorrect because the Voisins ring is not a characteristic finding in bone cysts, which are fluid-filled lesions that do not typically calcify in this manner.
**Option C:** This option is incorrect because the Voisins ring is not associated with bone metastases, which are typically irregular and not well-defined.

**Clinical Pearl / High-Yield Fact**
The Voisins ring is a classic radiographic sign of osteoid osteoma, and its presence should prompt consideration of this diagnosis in patients with bone pain and a characteristic radiographic appearance.

**Correct Answer: C. Osteoid osteoma**