**Question:** Commonest bone tumour is

A. Osteosarcoma
B. Chondrosarcoma
C. Ewing's sarcoma
D. Multiple Myeloma

**Correct Answer:** B. Chondrosarcoma

**Core Concept:**
Bone tumours are classified into primary and secondary tumours. Primary bone tumours are those that originate within the bone, while secondary bone tumours arise from malignant cells that have spread to the bone from a distant primary site. The most common primary bone tumour is chondrosarcoma, which develops from cartilage cells.

**Why the Correct Answer is Right:**
Chondrosarcoma is the most common primary bone tumour, accounting for approximately 25% of all primary bone malignancies. It develops from the cartilage cells (chondrocytes) within the cartilage matrix. In contrast to osteosarcoma (which also develops from bone cells), chondrosarcoma usually presents in older adults and tends to affect the long bones, particularly the femur and humerus. This is because chondrocytes have a higher risk of genetic mutations as they divide more frequently in these bones.

**Why Each Wrong Option is Incorrect:**

A. Osteosarcoma: Although osteosarcoma is a primary bone tumour, it develops from bone cells (osteoblasts) and is less common than chondrosarcoma (25% vs. 30-40%). Osteosarcoma primarily affects adolescents and young adults, whereas chondrosarcoma occurs in older adults. Osteosarcoma typically presents in the long bones, particularly the femur and tibia.

B. Chondrosarcoma is less common than osteosarcoma, making it the incorrect answer.
C. Ewing's sarcoma: Ewing's sarcoma is a rare primary bone malignancy that primarily affects children and teenagers. It is distinct from chondrosarcoma as it develops from neural crest cells and presents with a characteristic "ring" or "star"-shaped Ewing's sarcoma cells on biopsy.
D. Multiple myeloma: This is a plasma cell malignancy that occurs in the context of multiple myeloma or plasma cell dyscrasia. Multiple myeloma is a plasma cell malignancy involving plasma cells in the bone marrow, not a primary bone tumour.

**Clinical Pearls:**

1. Primary bone tumours can be classified as osteogenic (related to bone formation), osteolytic (destroys bone), or osteoblastic (promotes bone formation).
2. Osteosarcoma and chondrosarcoma are both osteogenic tumours, with a higher incidence in long bones and a higher incidence in children and adolescents compared to adults.
3. Ewing's sarcoma is a plasma cell dyscrasia, while multiple myeloma is a plasma cell malignancy involving plasma cells in the bone marrow. Both are unrelated to primary bone tumours.

**Why Multiple Choice Questions (MCQs) are important:** MCQs test students' understanding of core concepts and ability to recognize relationships among various medical conditions. They are crucial for assessing students' ability to differentiate between conditions and apply medical knowledge