Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) ankylosing spondylitis ?
    ### Response:
    Ankylosing spondylitis is a form of ongoing joint inflammation (chronic inflammatory arthritis) that primarily affects the spine. This condition is characterized by back pain and stiffness that typically appear in adolescence or early adulthood. Over time, back movement gradually becomes limited as the bones of the spine (vertebrae) fuse together.
    