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
A35 yr old lady with Normal PT and increased aPTT. 2 year back, she was operated for cholecystectomy & did not have any bleeding episode. What is next investigation for clinical diagnosis - 
Select and explain. 
A. Factor VIII assay, B. Anti viper venom assay, C. Platelet aggregation test, D. Ristocetin cofactor assay
B: Anti viper venom assay
Answer: B: Anti viper venom assay 
Explanation: None