**Core Concept**
The question is testing the ability to differentiate between a Gartner's cyst and a cystocele, which are two distinct **gynecological conditions**. A Gartner's cyst is a **benign cystic lesion** that arises from the remnants of the **mesonephric duct**, while a cystocele is a type of **pelvic organ prolapse** where the **bladder bulges** into the vagina.

**Why the Correct Answer is Right**
To differentiate between these two conditions, one must understand their **anatomical origins** and **clinical presentations**. Gartner's cysts are typically **asymptomatic** and **translucent**, while cystoceles present with **urinary symptoms** and a **bulge in the vagina**.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the **location** of the cyst can help differentiate between the two conditions.
**Option B:** This option is incorrect because the **size** and **consistency** of the cyst can also be used to differentiate.
**Option C:** This option is incorrect because **transillumination** can help distinguish between a Gartner's cyst and a cystocele.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Gartner's cysts are usually **asymptomatic** and **do not require treatment**, while cystoceles can cause significant **urinary symptoms** and may require **surgical intervention**.

**Correct Answer:** D. Transillumination test