Unilateral lower extremity edema is generally considered local lesions, so the left lower extremity swelling may be the left lower extremity varicose veins, the left lower extremity tansy (acute infectious disease, visible redness of the skin like paint), the left lower extremity venous thrombosis, and so on, need to be analyzed specifically. 1. Varicose veins of the left lower extremity: varicose veins of the lower extremity can be due to lower extremity blood return obstacles to lower extremity edema, can also be accompanied by hyperpigmentation, rash and other symptoms, lower extremity venous ultrasound and other tests can be used to assist in the definitive diagnosis. 2. Left lower extremity dengue (acute infection, redness of the skin like paint): dengue is mainly due to hemolytic streptococcus caused by superficial dermal lymphatic infections, redness, swelling, heat, pain in the infected area, accompanied by chills, chills, fever, and other symptoms, swabs from the infected area can be used to help clarify the diagnosis of bacterial culture and other tests. 3. Left lower extremity venous thrombosis: lower extremity venous thrombosis can be accompanied by swelling of the lower extremities, pain and/or tenderness, aggravated by activity, lower extremity venous ultrasound and other tests can assist in the definitive diagnosis. There are many causes of left lower extremity swelling, it is recommended to consult a doctor in time to make a clear diagnosis to avoid delaying the condition.