Review Question - QID 210047

A 55-year-old woman presents to the emergency room with right leg pain and swelling. She recently returned to the United States from a vacation in New Zealand. Her past medical history is notable for hypertension and generalized anxiety disorder. She takes hydrochlorothiazide and sertraline. She has a 10-pack-year smoking history and drinks alcohol socially. Her temperature is 98.8°F (37.1°C), blood pressure is 135/85 mmHg, pulse is 84/min, and respirations are 18/min. On examination, the circumference of her right calf is 3 cm larger than that of her left calf. A lower extremity ultrasound demonstrates a deep venous thrombosis in the femoral vein. She is discharged on low-molecular weight heparin with plans to follow-up in one week. However, 6 days after discharge, she presents to the emergency room again with a skin rash and new pain in her left leg. An image of her skin is shown in Figure A. A lower extremity ultrasound demonstrates a persistent deep venous thrombosis in the right femoral vein as well as a new deep venous thrombosis in the left popliteal vein. Which of the following is the most likely cause of this patient’s new symptoms?