## **Core Concept**
The superior laryngeal nerve is a branch of the vagus nerve (cranial nerve X) that provides sensory and motor innervation to the larynx. It divides into two branches: the internal laryngeal nerve and the external laryngeal nerve. The external branch of the superior laryngeal nerve primarily supplies the cricothyroid muscle.

## **Why the Correct Answer is Right**
The external branch of the superior laryngeal nerve provides motor innervation to the **cricothyroid muscle**. This muscle is one of the intrinsic muscles of the larynx and is responsible for tensing the vocal cords by adjusting the tension of the vocal cords. This action is crucial for changing the pitch of the voice. The cricothyroid muscle is the only intrinsic muscle of the larynx not supplied by the recurrent laryngeal nerve.

## **Why Each Wrong Option is Incorrect**
- **Option A:** The internal laryngeal nerve, not the external branch, provides sensory innervation to the mucous membrane of the larynx above the vocal cords.
- **Option B:** The recurrent laryngeal nerve supplies several intrinsic muscles of the larynx, including the thyroarytenoid, lateral cricoarytenoid, posterior cricoarytenoid, and arytenoid muscles, but not the cricothyroid muscle.
- **Option C:** (No specific detail provided, assuming incorrect based on the context).
- **Option D:** (No specific detail provided, assuming incorrect based on the context).

## **Clinical Pearl / High-Yield Fact**
A key clinical point to remember is that the external branch of the superior laryngeal nerve is at risk during thyroid surgery. Injury to this nerve can lead to vocal cord paralysis or weakness, manifesting as hoarseness of voice or difficulty in speaking. The cricothyroid muscle's unique innervation by the external branch of the superior laryngeal nerve, unlike the other intrinsic laryngeal muscles supplied by the recurrent laryngeal nerve, is a crucial anatomical detail.

## **Correct Answer:** C. cricothyroid muscle.