True regarding osteosarcoma is-
**Core Concept**
Osteosarcoma is a type of malignant bone tumor characterized by the uncontrolled proliferation of osteoid-producing cells. It is the most common primary malignant bone tumor in children and adolescents, often arising in the metaphyseal region of long bones.
**Why the Correct Answer is Right**
Osteosarcoma arises from the mesenchymal cells of the bone marrow, which differentiate into osteoblasts and produce osteoid. The tumor cells exhibit a characteristic "sunburst" or "crazy paving" appearance on radiographs due to the rapid deposition of osteoid. The most common site is the distal femur, followed by the proximal tibia. Osteosarcoma often metastasizes to the lungs, and the presence of pulmonary metastases is a poor prognostic indicator.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because osteosarcoma typically arises in the metaphyseal region of long bones, not the diaphysis.
**Option B:** This option is incorrect because while osteosarcoma can arise in the pelvis, it is not the most common site.
**Option C:** This option is incorrect because osteosarcoma is not typically associated with a good prognosis in the absence of metastases.
**Clinical Pearl / High-Yield Fact**
A classic radiographic feature of osteosarcoma is the presence of a "Codman triangle," which forms due to the raised periosteum and the underlying bone destruction.
**Correct Answer:** C.