Review Question - QID 109635

A 37-year-old previously healthy woman presents to the emergency room with right leg pain and difficulty breathing. She recently returned from a trip to Alaska and noticed her leg started to swell when she got home. Her medications include a multivitamin and oral contraceptives. She is diagnosed with a deep venous thrombosis complicated by a pulmonary embolism and started on anticoagulation. She remains stable and is discharged on the third hospital day with long-term anticoagulation. During the 2 month follow-up visit, the patient’s lab results are as follows:

Hemoglobin: 14 g/dL
Hematocrit: 44%
Leukocyte count: 5,000/mm^3 with normal differential
Platelet count: 300,000/mm^3
Prothrombin time: 23 seconds
Partial thromboplastin time (activated): 20 seconds
Bleeding time: 4 minutes

Which of the following factors is initially involved in the target pathway for her long-term treatment?