A 38-year-old woman with acute promyelocytic leukemia (APL) presents with fever, bleeding gums, and petechiae. Laboratory findings show platelet count 45,000/μL, PT 18 sec (control 12 sec), aPTT 52 sec (control 36 sec), and fibrinogen 85 mg/dL (normal 200–400 mg/dL). Which investigation is most specific for confirming disseminated intravascular coagulation (DIC)?