Deep Vein Thrombosis of the Common Femoral Vein

A 65-year-old woman with a history of treated breast cancer (on tamoxifen) presents with unilateral left calf swelling and warmth one week after a 14-hour flight. Her Wells score is 5 (high probability) and D-dimer is 4,800 ng/mL. Compression duplex ultrasound of the left lower extremity is shown. The structure marked **D** demonstrates non-compressibility, dilation, and echogenic intraluminal material with absent colour flow on Doppler. What is the most appropriate immediate management of this finding?