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: Gynaecology & Obstetrics
[{'content': 'Pathognomonic of ectopic pregnancy is : \nSelect and explain. \nA. Purple hemorrhagic pregnancy in lower vagina, B. Decidual casts, C. Chadwick sign, D. Postcoital hemorrhage', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Decidual casts', 'role': 'assistant'}]