**Core Concept**
The internal anal sphincter, a smooth muscle layer, is primarily controlled by the autonomic nervous system, whereas the external anal sphincter, a striated muscle layer, is controlled by the somatic nervous system. The pelvic nerve, a branch of the sacral spinal nerves, mediates parasympathetic innervation to the internal anal sphincter, facilitating its relaxation during defecation.

**Why the Correct Answer is Right**
The pelvic nerve carries parasympathetic fibers that stimulate the relaxation of the internal anal sphincter, allowing for the passage of stool. In this scenario, the internal anal sphincter exhibits normal tone but fails to relax in response to rectal distension, indicating a disruption in the parasympathetic input from the pelvic nerve. This is consistent with a lesion affecting the pelvic nerve, which would prevent the normal relaxation of the internal anal sphincter.

**Why Each Wrong Option is Incorrect**
**Option A:** The internal anal sphincter is not damaged, as it exhibits normal tone during the examination.
**Option B:** The external anal sphincter contracts in response to rectal distension, indicating intact somatic innervation from the pudendal nerve.
**Option D:** The pudendal nerve is responsible for somatic innervation to the external anal sphincter, not the internal anal sphincter, and is not directly involved in the relaxation of the internal anal sphincter.

**Clinical Pearl / High-Yield Fact**
The pelvic nerve plays a critical role in regulating the internal anal sphincter's relaxation, and damage to this nerve can lead to constipation or difficulty with defecation. This is an important consideration in patients with chronic constipation, as it may indicate a need for further evaluation of the pelvic nervous system.

✓ Correct Answer: C. Pelvic nerve