Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Pulmonary vein stenosis ?
    ### Response:
    Pulmonary vein stenosis is a very rare and serious condition in which there is a blockage in the blood vessels that bring oxygen-rich blood from the lungs back to the heart. This condition can be isolated to one vein, but often affects multiple veins. Stenosis occurs when there is an abnormal thickening and narrowing of the walls of the veins.
    