Below is an instruction from Human. Write a response.
    ### Instruction:
    What causes Pulmonary Embolism ?
    ### Response:
    Major Causes
                
Pulmonary embolism (PE) usually begins as a blood clot in a deep vein of the leg. This condition is called deep vein thrombosis. The clot can break free, travel through the bloodstream to the lungs, and block an artery.
                

    