Pulmonary embolism, illustration. Embolus (here a blood clot) blocking a pulmonary artery (upper right) in the lungs after travelling there from elsewhere in the body (the femoral vein, lower left). This is the most common form of pulmonary embolism, caused by an abnormal blood clot that has travelled from the leg to the lungs. An example is deep vein thrombosis (DVT). If not treated, a pulmonary embolism can be fatal.