**Core Concept**
Rectal incontinence is a condition where a person experiences involuntary loss of feces from the anal canal. It can be caused by damage to the muscles or nerves that control the anal sphincters. The anal sphincters are responsible for maintaining continence and preventing fecal incontinence.

**Why the Correct Answer is Right**
The internal anal sphincter is a smooth muscle layer that is responsible for maintaining continence at rest. It is a part of the involuntary nervous system and is controlled by the autonomic nervous system. Damage to the internal anal sphincter can lead to involuntary loss of feces, resulting in rectal incontinence. The internal anal sphincter is also responsible for the "resting tone" of the anal canal, which helps to prevent fecal leakage.

**Why Each Wrong Option is Incorrect**
* **Option A:** The external anal sphincter is a voluntary muscle that helps to control the anal canal during defecation. While it plays a role in maintaining continence, damage to the external anal sphincter would not typically cause rectal incontinence.
* **Option C:** The ischiococcygeus muscle is a part of the levator ani muscle group and helps to support the pelvic floor. While it plays a role in maintaining continence, damage to this muscle would not typically cause rectal incontinence.
* **Option D:** The pubococcygeus muscle is a part of the levator ani muscle group and helps to support the pelvic floor. While it plays a role in maintaining continence, damage to this muscle would not typically cause rectal incontinence.

**Clinical Pearl / High-Yield Fact**
The internal anal sphincter is responsible for the "resting tone" of the anal canal, which helps to prevent fecal leakage. Damage to this muscle can lead to rectal incontinence, and it is often involved in conditions such as anal fissures and fistulas.

**Correct Answer:** B. Internal anal sphincter