**Core Concept**
Hysterosalpingography (HSG) is a radiological imaging technique used to visualize the fallopian tubes and uterus. It is commonly employed to assess the patency of the fallopian tubes and to identify any abnormalities in the uterine cavity, which can contribute to infertility.

**Why the Correct Answer is Right**
A female patient presenting with infertility undergoes HSG, which reveals a **hydrosalpinx**. A hydrosalpinx is a condition where the fallopian tube becomes distended with fluid due to a blockage or obstruction, often resulting from a congenital anomaly or pelvic inflammatory disease. This condition can lead to infertility by creating an adverse environment for fertilization and embryonic development.

* The presence of fluid in the fallopian tube can also affect the implantation of a fertilized egg, leading to recurrent miscarriages or failed pregnancies.
* In addition, the fluid-filled fallopian tube can act as a conduit for bacterial infection, further compromising fertility.

**Why Each Wrong Option is Incorrect**
**Option A:** A **tubal ligation** is a surgical procedure where the fallopian tubes are cut or blocked, resulting in permanent sterility. This condition would not be visible on HSG and is typically diagnosed through surgical history or physical examination.

**Option B:** A **uterine septum** is a congenital anomaly where a thin wall of tissue separates the uterus into two distinct cavities. While a uterine septum can contribute to infertility, it would not be directly visible on a fallopian tube-focused imaging study like HSG.

**Option C:** A **tubal cyst** is an abnormal growth or cyst formation within the fallopian tube. While a tubal cyst could potentially impact fertility, it is not a characteristic finding in a hydrosalpinx, which is characterized by a fluid-filled, distended fallopian tube.

**Clinical Pearl / High-Yield Fact**
In patients with infertility, a hydrosalpinx can significantly affect the success rate of in vitro fertilization (IVF) treatments. Studies have shown that the presence of a hydrosalpinx can reduce the chances of successful implantation by up to 50%. Therefore, it is essential to identify and address this condition before proceeding with IVF.

**Correct Answer: D. Hydrosalpinx**