All open into middle meatus except
**Core Concept**
The **middle meatus** of the nasal cavity is a crucial area for drainage of the paranasal sinuses. It contains the **ethmoidal bulla** and the **semilunar hiatus**, which are key structures for the drainage of the anterior ethmoidal, frontal, and maxillary sinuses.
**Why the Correct Answer is Right**
The correct answer is related to the anatomical structures that drain into the middle meatus. The **frontal sinus**, **anterior ethmoidal sinus**, and **maxillary sinus** all drain into the middle meatus through the **semilunar hiatus**. The **posterior ethmoidal sinus** drains into the **superior meatus**, making it the exception.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because the frontal sinus does drain into the middle meatus.
**Option B:** Incorrect as the anterior ethmoidal sinus also drains into the middle meatus.
**Option C:** Incorrect since the maxillary sinus opens into the middle meatus as well.
**Clinical Pearl / High-Yield Fact**
Remember that the **ostiomeatal complex** is a critical area for sinus drainage, and any obstruction here can lead to sinusitis. The middle meatus is a key part of this complex.
**Correct Answer:** D. Posterior ethmoidal sinus.