Review Question - QID 218698

A 9-year-old girl presents to the emergency room with her mother febrile to 39.5°C and with a white blood cell count (WBC) of 19,500/mm3, an erythrocyte sedimentation rate (ESR) of 55 mm/hr, and a C-reactive protein (CRP) of 9.0 mg/dL. An MRI of her knee shows the pathology in Figure A, and she is taken urgently for operative debridement. If the cultured specimen intra-operatively reveals the most common causative organism, which of the following factors released by the organism is most likely responsible for the activation of prothrombin to thrombin that allows for clot formation and increases the patient's risk for deep vein thrombosis (DVT)?