**Core Concept**
The posterior part of the larynx contains the arytenoid cartilages, the aryepiglottic folds, and the posterior cricoid cartilage. Conditions affecting this region can cause significant respiratory and swallowing difficulties.

**Why the Correct Answer is Right**
The correct answer will be the condition that does not affect the posterior part of the larynx. The arytenoid cartilages are involved in vocal cord closure, and conditions affecting this area can lead to voice changes, respiratory distress, or difficulty swallowing. Conditions affecting the posterior part of the larynx can also lead to aspiration or respiratory failure.

**Why Each Wrong Option is Incorrect**
* **Option A:** Croup (laryngotracheobronchitis) typically affects the anterior part of the larynx, causing subglottic edema and respiratory distress. However, it can also affect the posterior part of the larynx, especially in severe cases. So, it's not the best option.
* **Option B:** Epiglottitis is an inflammation of the epiglottis, which is a structure located in the anterior part of the larynx. However, it can also involve the surrounding tissues, including the aryepiglottic folds, which are located in the posterior part of the larynx.
* **Option C:** Laryngomalacia is a congenital condition characterized by soft and floppy tissues in the larynx, which can affect the posterior part of the larynx, especially the arytenoid cartilages.

**Clinical Pearl / High-Yield Fact**
When assessing a patient with respiratory distress or difficulty swallowing, it's essential to examine the posterior part of the larynx, including the arytenoid cartilages and aryepiglottic folds, for signs of inflammation, edema, or other abnormalities.

**Correct Answer: D.**