This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pathology
[{'content': 'The most common source of pulmonary embolism is- \nSelect and explain. \nA. Amniotic fluid embolism, B. Renal artery embolism, C. Large veins of lower limb, D. Cardiothoracic surgery', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Pulmonary embolism\n\nPulmonary embolism is a blockage of pulmonary artery or one of its branches.\nPulmonary embolism is an extremely common and highly lethal condition that is a leading cause of death in all age\xa0groups.\n\nEtiology\n\nIn more than 95% of instances, venous emboli originating from deep leg vein thrombi above the level of knee i.e. in popliteal, femoral or iliac veins.\nAlthough the most common site of deep vein thrombosis is calf veins, calf veins thrombosis is itself associated with a low risk of embolism, but when the thrombosis progresses proximally to involve the larger veins above the leg then the risk becomes very high.\nSo, \xa0Most common site of DVT → Calf veins\n\nMost common source of pulmonary embolism →\xa0DVT of proximal veins of lower limb i.e., popliteal, femoral\xa0or iliac veins.\nPathophysiology of pulmonary embolism\n\nIf the size of embolus is large, it may be impacted at the bifurcation of main pulmonary artery →\xa0Saddle embolus, or may be passed out into the smaller, branching arterioles.\nMore commonly, there are multiple emboli, or a large embolus may be fragmented into many smaller emboli which are then impacted in a number of vessels, particularly affecting the lower lobes of lungs.\nRarely, Paradoxical embolism may occur by passage of an embolus from the right side of the heart into the left heart through ASD or VSD. In this way, pulmonary emboli may reach systemic circulation.', 'role': 'assistant'}]