A 25 year old man presents for further workup of an episode of venous thromboembolism. He developed deep vein thrombosis (DVT) of the left lower limb approximately 4 months ago. Prior to this, he had been sitting in one place for 12 hours, during an airplane flight. At that time, multiple investigations were performed, including a complete blood count, peripheral blood smear, and liver and renal profiles. No obvious cause for the thrombotic episode could be discerned. He was then started on heparin and warfarin, with the former discontinued once his INR reached satisfactory levels. The latter was continued for 3 months in total, and stopped 1 month ago. There is no family history of thrombotic disorders, and he was apparently healthy earlier, with unremarkable medical, surgical, and drug histories.