**Core Concept:** Episiotomy is a surgical incision made in the perineum during childbirth to facilitate delivery of the baby's head and prevent tearing. The perineal body is a fibromuscular band that extends from the anal sphincter to the anus, separating the anterior and posterior lips of the anus. The posterior lip is adjacent to the perineal body, and extending the episiotomy posteriorly can injure structures immediately posterior to it.

**Why the Correct Answer is Right:** When the episiotomy incision is extended posteriorly beyond the perineum, the injury can involve the rectovaginal septum, which is a thin connective tissue partition that separates the vagina from the rectum. The injury to the rectovaginal septum can lead to symptoms like faecal and urinary incontinence, pelvic pain, and erectile dysfunction in men.

**Why Each Wrong Option is Incorrect:**

A. Anal Sphincter: This option might seem tempting due to its proximity, but the injury would have caused severe bleeding, making it less likely.
B. Perineal Body: This option is incorrect because the injury would have been more likely to affect the rectovaginal septum, as it is the structure directly posterior to the perineal body.
C. Perineal Body: Similar to option B, this option is also incorrect because the injury would have affected the rectovaginal septum, which is posterior to the perineal body.
D. Rectum: While the rectum is adjacent to the rectovaginal septum, it is not the correct answer as the injury would be more likely to affect the rectovaginal septum due to its proximity and the specific injury mentioned (extension of episiotomy).

**Clinical Pearl:** The correct answer is the rectovaginal septum as it is the most likely structure to be injured due to its location and the described injury (extension of episiotomy). This injury can lead to symptoms like faecal and urinary incontinence, pelvic pain, and erectile dysfunction in men. A thorough examination and understanding of the anatomy of the perineal region are crucial for diagnosing and treating these injuries effectively.