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': '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 - \nSelect and explain. \nA. Factor VIII assay, B. Anti viper venom assay, C. Platelet aggregation test, D. Ristocetin cofactor assay', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]