Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Pyoderma gangrenosum ?
    ### Response:
    Pyoderma gangrenosum is a rare, destructive inflammatory skin disease of which a painful nodule or pustule breaks down to form a progressively enlarging ulcer. Lesions may occur either in the absence of any apparent underlying disorder or in association with other diseases, such as ulcerative colitis, Crohn's disease, polyarthritis (an inflammation of several joints together), gammopathy, and other conditions .
    