**Core Concept**
Bone in bone appearance is a characteristic radiological finding indicative of a specific type of bone tumor. It is caused by the presence of two distinct bone trabeculae within the same bone lesion, giving it a "bone within a bone" appearance on radiographs.

**Why the Correct Answer is Right**
This phenomenon is typically observed in osteosarcomas, a type of primary malignant bone tumor. The bone in bone appearance is due to the presence of two separate areas of bone formation within the same tumor, which can be caused by the presence of two different populations of tumor cells or by the tumor's ability to induce bone formation in different areas.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while osteoid osteomas can present with a characteristic "nidus" within the bone, they typically do not exhibit a bone in bone appearance.
**Option B:** This option is incorrect because enchondromas are benign cartilage tumors that do not typically induce bone formation, let alone a bone in bone appearance.
**Option C:** This option is incorrect because fibrous cortical defects are benign fibrous lesions that do not typically present with a bone in bone appearance.

**Clinical Pearl / High-Yield Fact**
The bone in bone appearance is a classic radiological finding in osteosarcomas, but it can also be seen in other types of bone tumors such as chondrosarcomas and Ewing's sarcomas.

**Correct Answer: C. Osteosarcoma**