The only abductor of vocal cord is
**Core Concept**
The question is testing the motor innervation and function of the laryngeal muscles, specifically the abductor of the vocal cord. The correct answer is related to the anatomy of the larynx and the specific muscle responsible for abducting the vocal cord.
**Why the Correct Answer is Right**
The only abductor of the vocal cord is the posterior cricoarytenoid muscle (PCA). This muscle is innervated by the superior laryngeal nerve (a branch of the vagus nerve) and is responsible for rotating the arytenoid cartilages, thereby abducting the vocal cords. This action is essential for breathing and preventing vocal cord closure during inhalation.
**Why Each Wrong Option is Incorrect**
* **Option A:** The thyroarytenoid muscle is a cricothyroid muscle antagonist and a tensor of the vocal cords, not an abductor. It is innervated by the recurrent laryngeal nerve.
* **Option B:** The lateral cricoarytenoid muscle is a tensor of the vocal cords, not an abductor. It is also innervated by the recurrent laryngeal nerve.
* **Option D:** The interarytenoid muscle (posterior interarytenoid muscle) is involved in closing the posterior part of the glottis, not abducting the vocal cords. It is innervated by the recurrent laryngeal nerve.
**Clinical Pearl / High-Yield Fact**
The posterior cricoarytenoid muscle is the only muscle responsible for opening the glottis, and its paralysis can lead to difficulty breathing and vocal cord closure.
**Correct Answer:** C. Posterior cricoarytenoid muscle.