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: Anatomy
[{'content': 'All of the following are components of vaginal sphincter, EXCEPT: \nSelect and explain. \nA. External urethral sphincter, B. Internal urethral sphincter, C. Bulbospongiosus, D. Pubovaginalis', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Four muscles compress the vagina and act as sphincters:PubovaginalisExternal urethral sphincterUrethrovaginal sphincterBulbospongiosusRef: Clinically Oriented Anatomy By Keith L. Moore, Ahur F. Dalley, II, Anne M. R. Agur, 2013, Page 389.', 'role': 'assistant'}]