**Core Concept**
The tensor of the vocal cord is a muscle responsible for tensing the vocal cord, which is essential for producing sound with varying pitch and volume. This muscle plays a crucial role in vocal cord function, particularly in adjusting the tension of the vocal cords to modulate voice production.

**Why the Correct Answer is Right**
The cricothyroid muscle is the tensor of the vocal cord, as it attaches to the thyroid cartilage and cricoid cartilage, and its contraction causes the vocal cord to stretch and tense. This tensing action increases the length and tension of the vocal cord, ultimately altering the pitch of the voice. The cricothyroid muscle is innervated by the external branch of the superior laryngeal nerve, which is a branch of the vagus nerve. This muscle's action is essential for fine-tuning the voice, allowing for subtle changes in pitch and volume.

**Why Each Wrong Option is Incorrect**
* **Option B:** The interarytenoid muscle is responsible for relaxing the posterior part of the vocal cord, allowing for closure of the posterior glottis, and is not the tensor of the vocal cord.
* **Option C:** The posterior cricoarytenoid muscle is the primary abductor of the vocal cord, responsible for opening the glottis, and is not involved in tensing the vocal cord.
* **Option D:** The lateral cricoarytenoid muscle is involved in adduction of the vocal cord, helping to bring the vocal cords together, but it does not function as the tensor of the vocal cord.

**Clinical Pearl / High-Yield Fact**
The cricothyroid muscle's action is essential for producing high-pitched sounds, and its weakness or paralysis can lead to a hoarse or breathy voice. In patients with vocal cord dysfunction or paralysis, identifying the specific muscle involved can guide treatment and rehabilitation strategies.

✓ Correct Answer: A. Cricothyroid