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 sphincter of the lower genitourinary tract in females, EXCEPT: 
Select and explain. 
A. Bulbospongiosus, B. External urethral sphincter, C. Internal urethral sphincter, D. Pubovaginalis
C: Internal urethral sphincter
Answer: C: Internal urethral sphincter 
Explanation: Muscles compress the vagina and act like sphincters: pubovaginalis, external urethral sphincter (especially its urethrovaginal sphincter pa), and bulbospongiosus. The internal urethral sphincter, which is formed from the detrusor muscle, is the upper and involuntary sphincter. Ref: Clinical and Surgical Anatomy, 2/e By Singh, 2006, Page 194; Clinically Oriented Anatomy by Keith L. Moore, Ahur F. Dalley, 1999, Page 372.