You are answering a USMLE-style medical question. Question: A 28-year-old woman with no significant past medical history presents to her primary care physician with increased menstrual bleeding over the past three months. She also notes easy bruising and bleeding from her gums when brushing her teeth. She is sexually active with multiple partners and has no history of intravenous drug use. Physical exam is remarkable for petechiae and scattered ecchymoses on the patient's bilateral upper and lower extremities. Urine beta-HCG is negative. Laboratory results are as follows: Hgb 13.0 g/dL, WBCs 6,000/mL, platelets 95,000/mL, PT 13.2s, aPTT 30s. Peripheral blood smear shows normocytic, normochromic red blood cells and few platelets with no morphologic abnormalities. Which diagnostic study should be performed next? Options: A) Serum factor VIII levels and von Willebrand factor activity B) Measurement of ADAMTS13 activity C) HIV and HCV testing D) Bone marrow aspiration Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.