There are many causes for swollen feet and ankles. Many are obvious
such as burns, fractures, sprains, strains and arthritis (osteoarthritis,
rheumatoid and gout) and pregnancy.
Less obvious would be due to medical conditions like congestive heart failure, kidney failure and liver failure and low thyroid. Some blood pressure and heart medications are relatively common causes.
A sudden swollen leg or ankle in the absence of injury could signal a blood clot in a leg or deep vein thrombosis.
More common is the case of swollen ankles and feet in someone who otherwise feels well or is in good health. In this setting venous insufficiency can be the culprit and the person may not have any varicose veins or spider veins. This diagnosis can only be determined by having an ultrasound examination of the lower legs in the standing position and this should be done by someone with training in the evaluation of the superficial venous system.