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
Normal size but non functioning uterus is usually associated with: 
Select and explain. 
A. Stenosis of the external os, B. Uterine synechiae, C. Partial agenesis of the vagina, D. Complete absence of vagina
B: Uterine synechiae
Answer: B: Uterine synechiae 
Explanation: Lets have a look at each option:
Option a: Stenosis of external os would result in hematometra, i.e. uterus would be large and non functioning (hence ruled out).
Option c: Partial agenesis of vagina 
In parital agenesis of vagina a segment of vagina may be atretic in the upper-third. It is often associated with hypoplasia or even absence of cervix. Uterus may be normal and functioning or malformed.
Option d: Complete absence of vagina is almost always associated with absence of uterus.
Hence by exclusion our answer is b i.e. uterine synechiae (Asherman syndrome)
In Uterine syechiae–the uterus does not function normally due to presence of adhesions size of uterus is normal.