**Core Concept**
In a patient who has undergone total thyroidectomy, respiratory stridor can be caused by compression or obstruction of the airway by a postoperative complication. This can occur when the surgical procedure inadvertently damages or removes the structures that support the trachea.

**Why the Correct Answer is Right**
The correct cause of respiratory stridor in this patient is likely due to a **posterior laryngeal nerve injury**, which can lead to paralysis of the cricothyroid muscle. This muscle plays a crucial role in maintaining the patency of the airway by preventing the posterior displacement of the cricoid cartilage and subsequent narrowing of the tracheal lumen. When the cricothyroid muscle is paralyzed, the cricoid cartilage can move posteriorly, causing a narrowing of the tracheal lumen and resulting in stridor.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not a known cause of respiratory stridor following total thyroidectomy. The **recurrent laryngeal nerve** is indeed crucial for vocal cord function, but its injury would more likely result in hoarseness or voice changes rather than stridor.
* **Option B:** **Tracheomalacia** is a condition characterized by weakness or softening of the tracheal cartilage, which can lead to airway collapse. However, tracheomalacia is not a direct result of total thyroidectomy and would be an unusual complication in this context.
* **Option C:** **Subglottic stenosis** is a narrowing of the tracheal lumen below the vocal cords, which can be caused by prolonged intubation or tracheostomy. However, this is not a common complication of total thyroidectomy.

**Clinical Pearl / High-Yield Fact**
In patients who have undergone total thyroidectomy, it is essential to assess the function of the posterior laryngeal nerve by evaluating the movement and tension of the vocal cords. This can be done using a laryngoscope, and any signs of posterior laryngeal nerve injury should be promptly reported to the surgical team.

**Correct Answer:** C.