Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) retroperitoneal fibrosis ?
    ### Response:
    Retroperitoneal fibrosis is a disorder in which inflammation and extensive scar tissue (fibrosis) occur in the back of the abdominal cavity, behind (retro-) the membrane that surrounds the organs of the digestive system (the peritoneum). This area is known as the retroperitoneal space.
    