A 52-year-old woman presents with a 10-year history of progressive leg heaviness and swelling, worse at the end of the day and relieved by elevation. On standing examination, the structure marked **A** (tortuous dilated superficial veins) is clearly visible and palpable on the medial aspect of both legs. Duplex ultrasound confirms reflux in the great saphenous vein. Which of the following pathophysiological mechanisms BEST explains the development of the structure marked **A** in this patient?