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
A 45 year old lady with normal PT and increased aPTT. About 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. Dilute russel viper venom assay, C. Platelet aggregation test, D. Ristocetin cofactor assay
B: Dilute russel viper venom assay
Answer: B: Dilute russel viper venom assay 
Explanation: Answer- B. Dilute russel viper venom assayDilute Russel viper venom test is one of the test to detect lupus anticoagulant.Lupus anticoagulant is associated with prolongation in PTT and thrombosis (no bleeding)