**Core Concept**
Para-ovarian cysts are a type of ovarian cyst that arises from the tissues surrounding the ovary, such as the mesosalpinx, mesovarium, or infundibulopelvic ligament. These cysts are often benign and can be found in a variety of locations around the ovary.

**Why the Correct Answer is Right**
Para-ovarian cysts are typically derived from the mesothelial cells lining these periovarian tissues. These cells can undergo metaplastic changes, leading to the formation of cysts. The exact mechanism is not fully understood, but it is thought to involve hormonal influences and genetic mutations. The location and origin of para-ovarian cysts can make them challenging to diagnose and treat.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because para-ovarian cysts are not typically derived from the ovarian stroma or surface epithelium.
**Option B:** This option is incorrect because para-ovarian cysts are not directly related to the fallopian tube or its epithelial lining.
**Option C:** This option is incorrect because para-ovarian cysts are not specifically associated with endometriosis or ectopic pregnancy.

**Clinical Pearl / High-Yield Fact**
Para-ovarian cysts can be distinguished from other types of ovarian cysts based on their location and radiological appearance. A key characteristic is their tendency to be located close to the ovary, but not directly attached to it.

**Correct Answer:** D. Mesothelial cells.