**Core Concept**
Uterine synechiae refer to adhesions within the uterus, often resulting from previous uterine surgery, infections, or other forms of uterine trauma. These adhesions can lead to a non-functional uterus, even if its size appears normal. This condition is also known as Asherman's syndrome.

**Why the Correct Answer is Right**
Uterine synechiae form when the uterine lining (endometrium) adheres to the uterine walls or other structures within the uterus. This adhesion can disrupt normal uterine function, leading to irregular menstrual cycles, infertility, and recurrent miscarriages. The presence of synechiae can be confirmed through hysteroscopy, a procedure where a doctor inserts a small camera into the uterus to visualize the interior. In cases of Asherman's syndrome, the uterine cavity may appear smaller than normal due to the presence of adhesions.

**Why Each Wrong Option is Incorrect**
**Option A:** Stenosis of the external os refers to a narrowing of the external cervical os, which can cause difficulties during childbirth but does not directly affect the function of the uterus. While stenosis may be associated with uterine synechiae, it is not the primary cause of a non-functioning uterus.

**Option C:** Partial agenesis of the vagina refers to a congenital condition where the vagina is underdeveloped. While this condition can be associated with other uterine abnormalities, it is not directly related to a non-functioning uterus.

**Option D:** Complete absence of vagina is a rare congenital condition where the vagina is not developed at all. This condition can be associated with other uterine abnormalities, but it is not directly related to a non-functioning uterus.

**Clinical Pearl / High-Yield Fact**
Asherman's syndrome is often associated with a history of uterine surgery, such as dilation and curettage (D&C) procedures. It is essential to consider Asherman's syndrome in patients with unexplained infertility, recurrent miscarriages, or irregular menstrual cycles, especially those with a history of uterine trauma.

**Correct Answer Line**
✓ Correct Answer: B. Uterine synechiae