This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pathology
Most common primary malignant bone tumour is: 
Select and explain. 
A. Malignant melanoma, B. Multiple myeloma, C. Osteosarcoma, D. Ewing's sarcoma
B: Multiple myeloma
Answer: B: Multiple myeloma 
Explanation: Multiple myeloma is the most common primary neoplasm of the skeletal system. 
The disease is a malignancy of plasma cells. Plasma cells are a subset of B cells, which are the producers of humoral immunity factors termed antibodies. However, if malignant transformation occurs in a single plasma cell, its clones produce only a single type of immunoglobulin, and electrophoresis demonstrates a monoclonal peak corresponding to this particular immunoglobulin. 
Multiple myeloma has been the prototype of monoclonal malignancies, in this case, of plasma cells.
Reference: Shafer’s Textbook of ORAL PATHOLOGY Eighth Edition page no 241