**Core Concept**
The pyriform sinus is a part of the laryngeal framework that forms a recess in the laryngeal ventricle. It is one of the three compartments of the laryngeal ventricle, along with the ventricle itself and the epiglottic recess.

**Why the Correct Answer is Right**
The pyriform sinus is bounded laterally by the aryepiglottic fold, which is a mucosal fold that connects the arytenoid cartilages to the epiglottis. This fold separates the pyriform sinus from the rest of the laryngeal ventricle. The aryepiglottic fold is a critical structure in the larynx, as it helps to separate the respiratory and digestive tracts.

**Why Each Wrong Option is Incorrect**
* **Option A:** The false vocal cord is not a lateral boundary of the pyriform sinus, as it is located above the true vocal cord and does not form a lateral boundary with the pyriform sinus.
* **Option B:** The arytenoid cartilages are not a lateral boundary of the pyriform sinus, as they are located medially and form a boundary with the posterior wall of the pyriform sinus.
* **Option C:** The epiglottis is not a lateral boundary of the pyriform sinus, as it is located anteriorly and does not form a lateral boundary with the pyriform sinus.

**Clinical Pearl / High-Yield Fact**
The pyriform sinus is a potential space that can become infected and form an abscess, known as a perichondritis, which can be a serious complication of laryngeal surgery.

**Correct Answer: D. Aryepiglottic fold**