Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Prurigo nodularis ?
    ### Response:
    Prurigo nodularis is a skin condition characterized by hard crusty lumps that itch intensely.  The exact cause of the condition is unknown. However, it can occur in isolation or as a result of repeated trauma to chronic pruritus (itching). Treatment for the condition can be challenging.
    