Yоu аre the cliniciаn in а primary care clinic. A 52-year-оld patient presents with a daily sensatiоn of heaviness in the lower legs that has been gradually worsening over the past several months. On physical examination, you note visible varicose veins, mild pitting edema, and a dusky discoloration of both ankles and feet. The patient denies recent trauma, fever, or acute pain. Which of the following is the most likely cause of these symptoms?