**Core Concept**
The maxillary sinus is the largest of the paranasal sinuses, and its drainage is crucial for maintaining normal respiratory function and preventing infection. The maxillary sinus opens into the nasal cavity through a small opening called the semilunar hiatus.

**Why the Correct Answer is Right**
The maxillary sinus drains its mucus into the nasal cavity through the semilunar hiatus, which is located in the middle meatus of the nasal cavity. This opening is formed by the uncinate process of the ethmoid bone and the bulla ethmoidalis. The mucus from the maxillary sinus then flows into the nasal cavity through this opening, where it can be swallowed or expelled.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the frontal sinus does not open into the maxillary sinus, but rather drains into the middle meatus of the nasal cavity through the frontonasal duct.
* **Option B:** This option is incorrect because the sphenoid sinus does not directly open into the maxillary sinus, but rather drains into the sphenoethmoidal recess of the nasal cavity.
* **Option C:** This option is incorrect because the ethmoid sinus does not open into the maxillary sinus, but rather drains into the middle meatus of the nasal cavity through multiple small openings.

**Clinical Pearl / High-Yield Fact**
The maxillary sinus is prone to infection due to its close proximity to the teeth and the nasal cavity. Patients with chronic maxillary sinusitis may present with symptoms such as facial pain, nasal congestion, and purulent nasal discharge.

**Correct Answer: C. The maxillary sinus opens into the middle meatus of the nasal cavity.**