You are answering a USMLE-style medical question. Question: A 41-year-old G3P1 woman presents with a sudden onset throbbing headache, tinnitus, nausea, and left-sided weakness. Patient has no significant past medical history and takes no medications. Her last two pregnancies ended with spontaneous abortions before the 10th week of gestation. No significant family history. Her vital signs include: blood pressure 130/90 mm Hg, pulse 58/min, respiratory rate 11/min, and temperature 36.8℃ (98.2℉). GCS is 14/15. Physical examination shows 3+ deep tendon reflexes and increased muscle tone in the left upper and lower extremities. Laboratory findings are significant for the following: Platelet count 230,000/mm3 Fibrinogen 3.5 g/L Activated partial thromboplastin time 70 s Thrombin time 34 s A non-contrast CT of the head is performed and shown in the picture. Which of the following would be the next best diagnostic step in this patient? Options: A) Mixing study B) INR C) Ristocetin-induced platelet aggregation test D) Clot retraction study Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.