**Core Concept:** Hormone-dependent tumours are those that are influenced by the secretion of specific hormones. In the context of bone tumours, we are considering tumours that are regulated by hormones that act on the bone tissue or its components.

**Why the Correct Answer is Right:** Osteosarcoma is a primary bone tumour that develops from bone-forming cells called osteoblasts. It is a hormone-dependent tumour because it is stimulated by parathyroid hormone (PTH), which is a key regulator of calcium and phosphorus homeostasis. High levels of PTH lead to increased bone resorption, subsequently promoting osteosarcoma growth.

**Why Each Wrong Option is Incorrect:**

A. Osteoid osteoma: This is a benign bone tumour characterized by the production of woven bone matrix. It is not hormone-dependent, as it does not involve the regulation by hormones like PTH.

B. Chondrosarcoma: This is another type of bone cancer that originates from cartilage-producing cells. It is not hormone-dependent, as it does not involve the regulation by hormones like PTH.

C. Ewing's sarcoma: This is a malignant bone tumour that develops from primitive neuroectodermal cells. It is not hormone-dependent, as it does not involve the regulation by hormones like PTH.

D. Osteoblastoma: This is a benign bone tumour that originates from osteoblasts, cells responsible for bone formation. It is not hormone-dependent, as it does not involve the regulation by hormones like PTH.

**Clinical Pearl:** Hormone-dependent tumours, like osteosarcoma, are crucial to understand as they represent a subgroup of tumours that can be influenced by therapeutic interventions targeting the involved hormones. This knowledge can guide clinical decision-making, treatment options, and prognosis of patients with hormone-dependent tumours.