**Pelkan's Spur**

**Core Concept**
Pelkan's spur is a characteristic histopathological feature seen in osteosarcoma, a type of primary bone cancer. It is a metaphyseal lesion that appears as a small, irregularly shaped, bony projection or spur, often associated with osteoid tissue.

**Why the Correct Answer is Right**
Pelkan's spur is typically seen in osteosarcoma due to the uncontrolled proliferation of osteoid-producing tumor cells. These cells produce excess osteoid tissue, which leads to the formation of a bony projection or spur. The spur is often seen in the metaphysis of long bones, particularly in the femur and tibia. The pathogenesis of Pelkan's spur is closely related to the aggressive behavior of osteosarcoma cells, which invade and destroy surrounding bone tissue.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Pelkan's spur is not typically seen in osteochondroma, a benign bone tumor that arises from the cartilage.
**Option B:** This option is incorrect because osteoid osteoma, a benign bone tumor, does not typically exhibit Pelkan's spur.
**Option C:** This option is incorrect because fibrous cortical defect, a benign bone lesion, does not typically exhibit Pelkan's spur.

**Clinical Pearl / High-Yield Fact**
Pelkan's spur is a crucial diagnostic feature of osteosarcoma, and its presence should raise suspicion for this aggressive bone malignancy. Histopathological examination of the affected bone tissue is essential for confirming the diagnosis of osteosarcoma.

**Correct Answer: C.**