**Core Concept**
A perilymph fistula is a type of inner ear disorder characterized by a leakage of perilymph, a fluid surrounding the membranous labyrinth, into the middle ear or the external environment. This leakage can cause a range of symptoms, including vertigo, hearing loss, and tinnitus.

**Why the Correct Answer is Right**
The most common site of perilymph fistula is the oval window, where the stapes bone connects the inner ear to the middle ear. This area is prone to fistula formation due to its thin, membranous structure and its proximity to the high-pressure environment of the middle ear. When the oval window is compromised, perilymph can leak into the middle ear, leading to symptoms associated with perilymph fistula.

**Why Each Wrong Option is Incorrect**
* **Option A:** The round window, while also a potential site for perilymph fistula, is less commonly involved than the oval window. The round window is a thicker, more robust structure that is less likely to develop a fistula.
* **Option B:** The cochlear aqueduct, a small canal connecting the subarachnoid space to the inner ear, is not a common site for perilymph fistula. While it can be involved in certain types of inner ear disorders, it is not typically associated with perilymph fistula.
* **Option D:** The endolymphatic sac, a structure involved in the regulation of inner ear pressure, is not a common site for perilymph fistula. While it can be involved in certain types of inner ear disorders, it is not typically associated with perilymph fistula.

**Clinical Pearl / High-Yield Fact**
The symptoms of perilymph fistula can be exacerbated by changes in pressure, such as those experienced during air travel or scuba diving. This is because the increased pressure can cause further leakage of perilymph into the middle ear.

**Correct Answer: C. Oval window.**