Single foot swelling, the following common causes: 1, foot sprain: after a sprain occurs in a single foot, there will be significant local swelling and pain, and the patient cannot walk with weight, but can only hold a crutch or limp. X-rays can sometimes reveal fractures and dislocations, while severe cases require MRI to further confirm whether there is damage to the ligaments. 2. Gout attack: Gout crystals can stimulate the soft tissues of a single foot causing local inflammation, and the patient will have clinical symptoms such as pain, swelling, limited activity, and may even develop high fever. 3. Degenerative osteoarthritis of the ankle joint: such as the joint between the tibia and talus, or The joint between the talus and the heel bone can degenerate, causing local swelling and difficulty in weight bearing. MRI can be taken to find out the location and extent of the lesion.