**Core Concept:** Sinuses are a series of air-filled spaces in the skull, primarily involved in humidity production and vibration dampening. When a toothache and sinusitis coexist, the involved sinus is usually the maxillary sinus.

**Why the Correct Answer is Right:** Toothache and sinusitis in the maxillary sinus are often linked due to the close anatomical relationship between the maxillary teeth and sinuses. The maxillary sinus is located between the posterior maxillary teeth and the nasal cavity, making it susceptible to infection or inflammation when dental problems occur.

**Why Each Wrong Option is Incorrect:**

A. Frontal sinus: This is incorrect because the frontal sinus is located more anteriorly, far away from the maxillary teeth.

B. Sphenoid sinus: Although present, sphenoid sinus is deeper and less likely to be affected by dental issues compared to the maxillary sinus.

C. Ethmoid sinus: Similar to the sphenoid sinus, ethmoid sinus is also relatively less affected by dental issues compared to maxillary sinus due to its location.

D. Oral cavity: This is incorrect because the cause of toothache must be in the oral cavity, not the sinus itself.

**Clinical Pearl:** When treating a patient with toothache and sinusitis, one must consider the dental cause of the toothache, as treating the sinus infection alone may not alleviate the patient's symptoms. In severe cases, endodontic treatment (root canal) or extraction of the involved tooth may be necessary to resolve the toothache and concomitant sinusitis.

**Correct Answer:** D. Oral cavity