CT-PA — Pulmonary Embolism Filling Defect

A 58-year-old Indian woman presents to the emergency department with acute dyspnea, pleuritic chest pain, and tachycardia (HR 118/min, RR 26/min). She had undergone total abdominal hysterectomy 5 days ago and was immobilized for 3 days post-operatively. CT pulmonary angiography (CTPA) is performed. The structure marked **A** in the diagram shows an intraluminal filling defect in the right main pulmonary artery. Which of the following is the MOST LIKELY source of the thrombus responsible for this imaging finding?