**Core Concept**
The perineal body is a fibromuscular structure located in the midline of the perineum, serving as an attachment point for several muscles and fascial layers. Injuries to this area can lead to damage to surrounding structures.

**Why the Correct Answer is Right**
During an episiotomy, if the incision extends posteriorly beyond the perineal body, it can easily compromise the structures located posterior to it. The main structure at risk is the anal sphincter complex, which includes the external and internal anal sphincters. The external anal sphincter is a voluntary muscle, whereas the internal anal sphincter is a smooth muscle under involuntary control.

**Why Each Wrong Option is Incorrect**
* **Option A:** The ischial tuberosity is a bony structure located more laterally in the pelvis and is not directly posterior to the perineal body.
* **Option B:** The anal canal is the terminal portion of the digestive tract, but it is not the specific structure that would be injured by an episiotomy extending beyond the perineal body.
* **Option C:** The coccyx (tailbone) is a bony structure located at the very distal end of the vertebral column, and while it is posterior to the perineal body, the question specifically asks about the structure injured by an episiotomy.

**Clinical Pearl / High-Yield Fact**
In cases of perineal trauma or episiotomy, it's essential to carefully assess the anal sphincter complex for any signs of injury, as this can lead to fecal incontinence or other long-term complications.

**Correct Answer:** C. Coccyx.