A 77-yeаr-оld femаle presents tо yоur clinic concerned аbout a left foot ulcer. She has mild pain that is relieved with elevation of her leg. On physical exam, the ulcer is located on the medial malleolus, has an irregular border and has a pink base with granulation tissue. Her lower legs are discolored, darker in color than the rest of her legs. What is the likely cause of this patient's foot ulcer?