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
All of the following are components of vaginal sphincter, EXCEPT: 
Select and explain. 
A. External urethral sphincter, B. Internal urethral sphincter, C. Bulbospongiosus, D. Pubovaginalis
B: Internal urethral sphincter
Answer: B: Internal urethral sphincter 
Explanation: 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.