External anal sphincter is innervated by:
**Core Concept**
The external anal sphincter is a striated muscle that plays a crucial role in voluntary control over defecation. Its innervation is unique compared to other pelvic muscles, which are primarily innervated by the autonomic nervous system.
**Why the Correct Answer is Right**
The external anal sphincter is innervated by the inferior rectal nerve, which is a branch of the pudendal nerve. The pudendal nerve originates from the S2-S4 nerve roots and provides motor innervation to the external anal sphincter, allowing for voluntary control over defecation. This is in contrast to the internal anal sphincter, which is innervated by the sympathetic nervous system.
**Why Each Wrong Option is Incorrect**
**Option A:** The superior rectal nerve is a branch of the inferior mesenteric plexus and primarily provides parasympathetic innervation to the rectum, not the external anal sphincter.
**Option B:** The obturator nerve primarily innervates the adductor muscles of the thigh and is not involved in the innervation of the external anal sphincter.
**Option C:** The ilioinguinal nerve provides sensory innervation to the skin of the inguinal region and is not involved in the motor innervation of the external anal sphincter.
**Clinical Pearl / High-Yield Fact**
The pudendal nerve is also responsible for innervating the urethral sphincter and the muscles of the perineum, highlighting its critical role in maintaining continence.
**Correct Answer: D. The pudendal nerve.**