Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Ankylosing spondylitis ?
    ### Response:
    Ankylosing spondylitis (AS) is a type of chronic, inflammatory arthritis that mainly affects the spine. It usually begins with inflammation of the joints between the pelvic bones and spine, gradually spreading to the joints between the vertebrae. Signs and symptoms usually begin in adolescence or early adulthood and may include back pain and stiffness.
    