**Core Concept**
The question requires knowledge of the anatomy of the perineal region and the structures that lie posterior to the perineal body. The perineal body is a fibromuscular mass located in the midline of the perineum, and it serves as an attachment point for several muscles and fasciae.

**Why the Correct Answer is Right**
An episiotomy extended posteriorly beyond the perineal body would likely injure the anal sphincter complex, which includes the external anal sphincter and the internal anal sphincter. These muscles are located posterior to the perineal body and play a crucial role in maintaining anal continence. The anal sphincter complex is also richly innervated, which makes it a sensitive area.

**Why Each Wrong Option is Incorrect**
* **Option A:** The levator ani muscle is a major muscle of the pelvic floor, but it is not located posterior to the perineal body. It is a more lateral structure that plays a crucial role in supporting the pelvic organs.
* **Option B:** The rectum is a hollow, muscular tube that is located posterior to the perineal body, but it is not the specific structure that would be injured by an episiotomy extended posteriorly. The rectum is a more posterior structure than the anal sphincter complex.
* **Option C:** The ischiocavernosus muscle is a muscle of the superficial perineal pouch, but it is not located posterior to the perineal body. It is a more lateral structure that plays a role in erection.

**Clinical Pearl / High-Yield Fact**
It is essential for healthcare providers to be aware of the anatomy of the perineal region and the potential risks associated with episiotomy, particularly in the context of a second- or third-degree tear.

**Correct Answer: C. Ischiocavernosus muscle.