A 45-year-old man is admitted to the hospital with breathing problems. During physical examination the patient shows signs of airway obstruction. A CT scan examination reveals a nasal polyp obstructing the airway. Drainage from which of the following structures is also obstructed?

**Core Concept**
The nasal cavity is a complex structure with multiple passageways connecting to the paranasal sinuses. The maxillary sinus is one of the most anterior sinuses, located in the maxillary bone, and drains into the nasal cavity through the semilunar hiatus.

**Why the Correct Answer is Right**
The correct answer is B. Maxillary sinus because the nasal polyp obstructing the airway is likely located in the nasal cavity, which is the common drainage path for all paranasal sinuses. The maxillary sinus has a relatively short drainage pathway to the nasal cavity, making it more susceptible to obstruction by nasal polyps. The semilunar hiatus, where the maxillary sinus drains, is located near the middle meatus of the nasal cavity, which is often affected by nasal polyps.

**Why Each Wrong Option is Incorrect**
**Option A:** The sphenoid sinus drains into the sphenoethmoidal recess, which is located in the superior meatus of the nasal cavity. This is a less common site for nasal polyps.
**Option C:** The ethmoidal sinuses drain into the middle meatus of the nasal cavity, but they are smaller than the maxillary sinus and less likely to be involved in airway obstruction.
**Option D:** The frontal sinus drains into the middle meatus of the nasal cavity through the frontonasal duct, but it is less likely to be affected by nasal polyps compared to the maxillary sinus.

**Clinical Pearl / High-Yield Fact**
Nasal polyps often involve the middle meatus of the nasal cavity, which is a common site for drainage of the maxillary sinus. This is why the maxillary sinus is more likely to be affected by nasal polyps compared to other paranasal sinuses.

**✓ Correct Answer: B. Maxillary sinus**

✓ Correct Answer: B. Maxillary sinus