Swelling of the back of the left foot is commonly caused by the following reasons: First, trauma factors. A sprain of the left foot or a sprain of the ankle joint, especially a fracture, can lead to significant swelling of the back of the left foot, accompanied by painful symptoms. The patient’s history of trauma is relatively clear, and the presence of fracture can be found by taking X-ray film. Second, acute attack of gout, high serum uric acid will lead to gout crystals, which will be deposited under the skin on the back of the foot, thus inducing acute redness, swelling, pain and other clinical symptoms, and the patient cannot even walk with weight, and anti-gout treatment is needed to relieve it. Third, elderly patients with serious medical diseases, such as heart failure and renal failure, can lead to a significant increase in blood volume, and subcutaneous soft tissues are prone to edema, especially the dorsum of the foot. Therefore, they will show symptoms of swelling on the dorsum of the left foot, which sometimes can also involve the lower leg.