**Core Concept**
The ischiorectal fossa, also known as the ischioanal fossa, is a potential space in the pelvic region that contains fatty tissue and is bounded by several structures. The boundaries of the ischiorectal fossa are important for identifying and accessing the underlying structures in the pelvic region.

**Why the Correct Answer is Right**
The lateral boundary of the ischiorectal fossa is formed by the obturator internus muscle and its fascia. The obturator internus muscle is a flat, fan-shaped muscle that originates from the inner surface of the obturator membrane and the margins of the obturator foramen. Its fascia, which is a layer of connective tissue, forms the lateral boundary of the ischiorectal fossa. This boundary is important for separating the ischiorectal fossa from the pelvic cavity and the surrounding structures.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the obturator internus muscle and its fascia, not the obturator membrane, form the lateral boundary of the ischiorectal fossa.
* **Option B:** This option is incorrect because the levator ani muscle and its fascia form the medial boundary of the ischiorectal fossa, not the lateral boundary.
* **Option D:** This option is incorrect because the sacrotuberous and sacrospinous ligaments form the posterior boundary of the ischiorectal fossa, not the lateral boundary.

**Clinical Pearl / High-Yield Fact**
The ischiorectal fossa is a potential space that can become infected, leading to an abscess or fistula. The boundaries of the ischiorectal fossa are important for identifying and accessing the underlying structures in the pelvic region, and for performing surgical procedures in this area.

**Correct Answer: C. Obturator internus muscle and its fascia.**