Deep vein thrombosis (DVT) of the lower extremities may lead to pulmonary embolism (pulmonary infarction) or post-deep vein thrombosis syndrome, which may lead to death or disability in severe cases, so timely and accurate diagnosis of DVT of the lower extremities is particularly important. Etiology: Risk factors for DVT include smoking, advanced age, obesity, bed rest, malignancy, previous history of thromboembolism, trauma, travel, propensity to thrombosis (congenital anticoagulation factor deficiency, etc.), pregnancy and other factors such as oral administration of certain drugs such as birth control pills. Clinical manifestations: Mild cases may be asymptomatic, while severe cases may present with bruised or whitish swelling of the femur that endangers the presence of the limb. Common symptoms include suffocation, pain (significantly worse with walking), swelling, skin color change (cyanosis or reddening), limb hypertonia, limb tenderness, fever, superficial venous dilatation, and pain behind the calf during foot dorsiflexion. Laboratory tests and imaging tests: D-dimer and C-reactive protein can be increased in the acute stage, and negative D-dimer can basically exclude deep vein thrombosis; imaging tests include venous ultrasound and venography, the former is non-invasive, does not require contrast agent, and has been widely carried out in clinical practice. In addition, the etiology of DVT needs to be examined, such as the presence of anticoagulation factor deficiency, genetic defects and antiphospholipid syndrome, malignant tumor, etc.