## Core Concept
The larynx, or voice box, contains several intrinsic muscles that regulate voice production and breathing. These muscles are primarily innervated by the recurrent laryngeal nerve, a branch of the vagus nerve (cranial nerve X). However, one muscle of the larynx is supplied by a different branch of the vagus nerve.

## Why the Correct Answer is Right
The cricothyroid muscle is the only intrinsic muscle of the larynx not supplied by the recurrent laryngeal nerve. Instead, it is innervated by the external branch of the superior laryngeal nerve, another branch of the vagus nerve. This muscle is responsible for tensing the vocal cords, which affects pitch during speech and singing.

## Why Each Wrong Option is Incorrect
* **Option A:** The posterior cricoarytenoid muscle is one of the intrinsic muscles of the larynx and is supplied by the recurrent laryngeal nerve. It is the only muscle that can open (abduct) the glottis.
* **Option B:** The lateral cricoarytenoid muscle is also supplied by the recurrent laryngeal nerve and helps in closing the glottis and in adjusting the pitch of the voice.
* **Option C:** The arytenoid muscle, or interarytenoid muscle, receives its nerve supply from the recurrent laryngeal nerve as well. It helps in closing the posterior part of the rima glottidis.

## Clinical Pearl / High-Yield Fact
A key clinical point to remember is that the recurrent laryngeal nerve can be damaged during surgical procedures, such as thyroidectomy, leading to vocal cord paralysis. The cricothyroid muscle's separate innervation by the external branch of the superior laryngeal nerve is crucial for preserving some voice function even when the recurrent laryngeal nerve is damaged.

## Correct Answer: C. cricothyroid.