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
Best assay for deficiency of von Willebrand factor is- 
Select and explain. 
A. Bleeding time, B. APTT, C. BT + APTT, D. BT + APTT + vWF-ristocetin factor assay
D: BT + APTT + vWF-ristocetin factor assay
Answer: D: BT + APTT + vWF-ristocetin factor assay 
Explanation: None