This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Medicine
[{'content': 'Best assay for deficiency of von Willebrand factor is- \nSelect and explain. \nA. Bleeding time, B. APTT, C. BT + APTT, D. BT + APTT + vWF-ristocetin factor assay', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]