Below is an instruction from Human. Write a response.
    ### Instruction:
    What causes Poland syndrome ?
    ### Response:
    What causes Poland syndrome? The cause of Poland syndrome is unknown. Most evidence supports the idea that something abnormal happens during the sixth week of fetal development. This event most likely involves the vascular (blood and lymph) system. Speculations include: An interruption of the embryonic blood supply of the arteries that lie under the collarbone (subclavian arteries).
    