**Core Concept**
The appendix of the testis, also known as the hydatid of Morgagni, is a small, vestigial structure located at the upper pole of the testis. It is a remnant of a peritoneal diverticulum that formed during fetal development.

**Why the Correct Answer is Right**
The appendix of the testis is thought to be a remnant of the Müllerian duct, which is a embryonic structure that gives rise to the female reproductive tract. In males, the Müllerian duct regresses due to the action of anti-Müllerian hormone (AMH) produced by the Sertoli cells of the testis. However, a small portion of the duct can persist and form the appendix of the testis. This remnant is usually small and contains a small amount of fibrous tissue and sometimes a small amount of fluid.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the appendix of the testis is not a remnant of the Wolffian duct, which is a different embryonic structure that gives rise to the male reproductive tract.
**Option B:** This option is incorrect because the appendix of the testis is not a remnant of the gubernaculum, which is a fibrous cord that helps to guide the testis into its final position in the scrotum.
**Option C:** This option is incorrect because the appendix of the testis is not a remnant of the epididymis, which is a tube-like structure that stores and transports sperm.

**Clinical Pearl / High-Yield Fact**
It's worth noting that the appendix of the testis can sometimes be confused with a testicular tumor, so it's essential to distinguish between the two on clinical examination. A small, painless nodule at the upper pole of the testis is often a normal appendix of the testis.

**Correct Answer: D. Remnant of the Müllerian duct.