Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) alkaptonuria ?
    ### Response:
    Alkaptonuria is an inherited condition that causes urine to turn black when exposed to air. Ochronosis, a buildup of dark pigment in connective tissues such as cartilage and skin, is also characteristic of the disorder. This blue-black pigmentation usually appears after age 30.
    