Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Fibrous dysplasia ?
    ### Response:
    Fibrous dysplasia is a skeletal disorder that is characterized by the replacement of normal bone with fibrous bone tissue. It may involve one bone (monostotic) or multiple bones (polyostotic). Fibrous dysplasia can affect any bone in the body. The most common sites are the bones in the skull and face, the long bones in the arms and legs, the pelvis, and the ribs.
    