## **Core Concept**
The closure of the glottis is a critical function that prevents foreign particles from entering the airway during swallowing or other situations. This function is primarily controlled by the laryngeal muscles, which are innervated by the recurrent laryngeal nerve and the superior laryngeal nerve, both branches of the vagus nerve (cranial nerve X). The muscles involved work together to bring the vocal cords together.

## **Why the Correct Answer is Right**
The correct answer involves understanding which muscles are responsible for the closure of the glottis. The primary muscles involved in closing the glottis are:
- **Thyroarytenoid muscle**: Helps in relaxing the vocal cords and can contribute to closure.
- **Lateral cricoarytenoid muscle**: Rotates the arytenoid cartilages, bringing the vocal cords together.
- **Posterior cricoarytenoid muscle** is actually the only muscle responsible for opening (abducting) the glottis by rotating the arytenoid cartilages outward, which separates the vocal cords.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is not provided, but typically muscles like thyroarytenoid and lateral cricoarytenoid are involved in closure.
- **Option B:** Similarly, another muscle involved in closure would be listed here.
- **Option C:** A third muscle involved in closure would be mentioned.
- **Option D: Posterior cricoarytenoid**: This muscle is the only one that abducts the vocal cords, thereby opening the glottis. It does not contribute to the closure of the glottis.

## **Clinical Pearl / High-Yield Fact**
A key clinical point to remember is that during endotracheal intubation, the posterior cricoarytenoid muscle's function can be affected, leading to vocal cord paralysis or weakness. The cricothyroid muscle, which is not involved in glottis closure but in tensing the vocal cords, is innervated by the external branch of the superior laryngeal nerve.

## **Correct Answer: D. Posterior cricoarytenoid**