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
Transverse vaginal septum corresponds to: 
Select and explain. 
A. External Os, B. Vesical neck, C. Bladder base, D. Hymen
A: External Os
Answer: A: External Os 
Explanation: If there is a disorder in fusion of downgrowing Mullerian duct and upgrowing derivative of urogenital sinus, it results in transverse vaginal septum which causes imperforate vagina (or vaginal agenesis).
In a series reported:
• 46% septa were located in upper part.
• 40% septa were located in middle part.
• 14% septa were located in lower part.
The upper part corresponds to external os, therefore it is the option of choice. For further details on transverse vaginal septum see the preceding text.