**Core Concept**
The recurrent laryngeal nerve is a branch of the vagus nerve that supplies the intrinsic muscles of the larynx, except for one muscle, which is responsible for tension in the vocal cords. This muscle plays a crucial role in regulating the pitch and volume of the voice.

**Why the Correct Answer is Right**
The cricothyroid muscle is the only intrinsic muscle of the larynx that is not supplied by the recurrent laryngeal nerve. Instead, it is innervated by the external branch of the superior laryngeal nerve, a branch of the vagus nerve. The cricothyroid muscle is responsible for tensing the vocal cords, thereby increasing the pitch of the voice. This muscle is essential for producing high-pitched sounds and for maintaining vocal cord tension during vocalization.

**Why Each Wrong Option is Incorrect**
* **Option A:** The posterior cricoarytenoid muscle is the only abductor of the vocal cords and is supplied by the recurrent laryngeal nerve.
* **Option B:** The thyroarytenoid muscle is one of the intrinsic muscles of the larynx that is supplied by the recurrent laryngeal nerve.
* **Option D:** The arytenoid muscle is also supplied by the recurrent laryngeal nerve and is involved in the closure of the glottis.

**Clinical Pearl / High-Yield Fact**
The cricothyroid muscle is the only intrinsic muscle of the larynx that is not supplied by the recurrent laryngeal nerve, and its innervation is instead provided by the external branch of the superior laryngeal nerve.

**Correct Answer:** C. Cricothyroid muscle.