Okay, let's tackle this question. The patient has toothache and sinusitis. The question is asking which sinus is involved. Hmm, I need to recall the anatomy of the sinuses and their relation to teeth.

First, the maxillary sinuses are located in the maxilla, which is the upper jaw. The roots of the upper molars are close to the floor of the maxillary sinus. So, if there's an infection in the upper molars, like a dental abscess, it could spread to the maxillary sinus. That makes sense because the maxillary sinus is the most commonly affected in cases of dental infections.

Now, the other sinuses: frontal, ethmoid, and sphenoid. The frontal sinuses are in the forehead area, so they're not near the teeth. The ethmoid sinuses are between the eyes, near the nasal cavity, but they are more involved in nasal congestion and maybe allergies. The sphenoid is behind the ethmoid, near the base of the skull, and infections there are rare.

So the most likely answer is the maxillary sinus. The key here is the proximity of the upper molars to the maxillary sinus. If the toothache is from a molar, especially the upper ones, it's a classic presentation of a dental infection leading to sinusitis. The question probably lists the options as maxillary, frontal, ethmoid, sphenoid. So the correct answer is maxillary.

**Core Concept**
The maxillary sinus, located inferior to the orbit and adjacent to the roots of upper molars, is anatomically linked to dental infections. Dental abscesses in upper molars can directly spread to this sinus due to their proximity, making it the most common site of odontogenic sinusitis.

**Why the Correct Answer is Right**
The maxillary sinus (Option C) is the largest paranasal sinus and lies in the maxilla, directly below the upper molars. Infections from these teeth, such as apical periodontitis or dental abscesses, can spread inferiorly into the sinus due to their shared anatomical space. This is a classic example of odontogenic sinusitis, where dental pathology leads to sinus inflammation.

**Why Each Wrong Option is Incorrect**
**Option A (Frontal sinus):** Located in the forehead, it is anatomically distant from the maxillary teeth and not involved in odontogenic infections.
**Option B (Ethmoid sinus):** Situated between the eyes and nasal cavity, it is more associated with viral sinusitis or allergic rhinitis than dental sources.
**Option D (Sphenoid sinus):** Posterior and deep, it rarely causes symptoms unless infected in chronic cases unrelated to teeth.

**Clinical Pearl / High-Yield Fact**
Upper molar infections (e.g., apical abscess) almost always involve the maxillary sinus. Remember the mantra: “Toothache + facial pain = maxillary sinusitis.” Always consider sinus imaging in dental cases with persistent symptoms.

**Correct Answer: C. Maxillary sinus**

✓ Correct Answer: A. Maxillary