Cricothyroid muscle is supplied by
**Core Concept**
The cricothyroid muscle is involved in voice production by altering the tension of the vocal cords. It is the only intrinsic laryngeal muscle not supplied by the recurrent laryngeal nerve. The underlying principle here involves the **innervation of laryngeal muscles** and the specific role of the **cricothyroid muscle** in vocal cord function.
**Why the Correct Answer is Right**
The correct answer is related to the unique innervation of the cricothyroid muscle, which is supplied by the **external branch of the superior laryngeal nerve**. This nerve is a branch of the **vagus nerve (CN X)** and is responsible for the motor innervation of the cricothyroid muscle, allowing for the adjustment of vocal cord tension and pitch.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it does not specifically relate to the known innervation of the cricothyroid muscle.
**Option B:** Incorrect as it is not associated with the cricothyroid muscle's supply.
**Option C:** Incorrect because the recurrent laryngeal nerve supplies all other intrinsic laryngeal muscles except the cricothyroid.
**Option D:** Incorrect as it is not the nerve responsible for the cricothyroid muscle's innervation.
**Clinical Pearl / High-Yield Fact**
Remembering that the cricothyroid muscle is the only intrinsic laryngeal muscle not supplied by the recurrent laryngeal nerve but by the **external branch of the superior laryngeal nerve** is crucial for understanding laryngeal function and pathology.
**Correct Answer:** Correct Answer: D. External branch of the superior laryngeal nerve.