Varicose veins and ulcer, illustration. An ulcer is an open sore on the skin that has failed to heal. This ulcer has arisen as a complication of varicose veins (blue). Varicose veins are tortuous distended veins caused by defective valves in the blood vessels. They cause poor blood flow, which can lead to areas of tissue death due to a lack of oxygen. The affected area is dressed, and antibiotics are taken to prevent infection of the ulcer.