This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Haematology, 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 episoe. 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: The dilute Russell viper venom test is a commonly used screening test for LA. It is called the dilute Russell viper venom time test because the phospholipid reagent is diluted such that its concentration becomes rate limiting; thus, it becomes a sensitive assay Ref Davidson 23rd edition pg 283