Popliteal Deep Vein Thrombosis

A 58-year-old woman presents with 4 days of progressive right calf swelling and posterior knee pain following a 14-hour flight and recent initiation of combined oral contraceptives. She has class II obesity, prior breast cancer (on tamoxifen), and a family history of venous thromboembolism. Clinical Wells score is 4 (likely DVT). Compression venous ultrasound is performed. The structure marked **A** in the diagram—the popliteal vein—is visibly dilated and contains echogenic intraluminal material. When transducer compression is applied, the vein walls fail to approximate despite pressure sufficient to deform the adjacent popliteal artery. Which of the following best describes the diagnostic significance of this finding?