This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Bleeding disorder, Subject name: Pathology
[{'content': 'Investigation to distinguish between pregnancy acquired hemophilia A and lupus anticoagulant? \nSelect and explain. \nA. Factor 8 assay, B. dRVVT, C. von Willlebrand Factor assay, D. aPTT', 'role': 'user'}, {'content': "Answer: B \nExplanation: In some cases of pregnancy there is antibody formation against the clotting factors, paicularly against factor 8. Dilute Russell's viper venom time- -In-vitro diagnostic test is based on the ability of the venom of the Russell's viper to induce thrombosis. -The venom contains some substances (RVV-V and RVV-X) which directly activate factor V and factor X which turns prothrombin into thrombin in the presence of phospholipid and calcium. -This makes the test sensitive to the presence of lupus anticoagulants, because these antibodies interfere with the clot-promoting role of phospholipid in vitro, and their presence results in a prolonged clotting time. NOTE - the dilute Russell's viper venom test and the hexagonal-phase phospholipids test will be Negative in patients with an acquired inhibitor and Positive in patients with lupus anticoagulants.", 'role': 'assistant'}]