Deep Vein Thrombosis (Common Femoral Vein)

A 56-year-old woman presents 10 days after a long-haul international flight with right calf swelling (4 cm larger than left), pitting edema, and calf tenderness. She takes combined oral contraceptives and smokes. Wells DVT score is 4 (HIGH probability). Compression venous duplex ultrasound is performed. The structure marked **A** in the diagram is dilated and non-compressible under direct probe pressure, contains echogenic intraluminal material, and shows absent color Doppler flow. What is the most appropriate IMMEDIATE next step in management?