This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Abdomen and pelvis, Subject name: Anatomy
[{'content': 'What is the shape of external os in nulliparous female ? \nSelect and explain. \nA. circular, B. transvers, C. veical, D. oval', 'role': 'user'}, {'content': "Answer: A \nExplanation: In a nulliparous woman, i.e. a woman who has not borne children, the external os is small and circular. However, in multiparous women, the external os is bounded by anterior and posterior lips, both of which are in contact with the posterior wall of the vagina. Ref : B D Chaurasia's Human Anatomy , seventh edition, volume 2, pg.no.421.", 'role': 'assistant'}]