**Core Concept**
A Gartner's duct cyst or a Bartholin's cyst is not the correct diagnosis in this case, given the location and the cyclic nature of the swelling. The correct diagnosis involves a congenital anomaly of the female reproductive system, specifically a cystic structure that arises from the remnants of the mesonephric ducts.

**Why the Correct Answer is Right**
The patient's symptoms suggest a cystic structure that is influenced by hormonal changes, specifically the increase in progesterone levels before menstruation. This is consistent with a Gartner's duct cyst, which is a congenital anomaly that arises from the remnants of the mesonephric ducts in the female reproductive system. The mesonephric ducts are present in the embryo and usually regress, but in some cases, remnants can persist and form cysts.

**Why Each Wrong Option is Incorrect**
* **Option A:** A Bartholin's cyst is a different diagnosis, typically presenting as a swelling on the vulva, not on the posterior vaginal wall.
* **Option B:** A Gartner's duct cyst is the correct answer, given the location and the cyclic nature of the swelling.
* **Option C:** A vaginal inclusion cyst is a different diagnosis, typically presenting as a swelling on the vaginal wall, but not necessarily related to hormonal changes.
* **Option D:** A vaginal fibroma is a different diagnosis, typically presenting as a firm, non-cyclic swelling on the vaginal wall.

**Clinical Pearl / High-Yield Fact**
Gartner's duct cysts are usually diagnosed in premenopausal women and can be associated with other congenital anomalies, such as renal anomalies.

**Correct Answer:** B. A Gartner's duct cyst is a congenital anomaly that arises from the remnants of the mesonephric ducts in the female reproductive system.