Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Retroperitoneal fibrosis ?
    ### Response:
    Retroperitoneal fibrosis is a slowly progressive disorder in which the tubes that carry urine from the kidneys to the bladder (ureters) and other abdominal organs are blocked by a fibrous mass and inflammation in the back of the abdomen. The disorder may cause chronic unilateral obstructive uropathy or chronic bilateral obstructive uropathy.
    