**Core Concept**
The maxillary sinus is one of the paranasal sinuses, a group of air-filled cavities located within the bones of the face. The maxillary sinus is the largest of these sinuses and plays a crucial role in the drainage of the nasal cavity. Understanding the position and orientation of the maxillary sinus is essential for diagnosing and treating sinus-related disorders.

**Why the Correct Answer is Right**
The apex of the maxillary sinus is directed medially and slightly posteriorly, pointing towards the zygomatic bone. This is due to the anatomical relationship between the maxillary sinus and the surrounding structures. The maxillary sinus is situated in the maxillary bone, which forms the lateral wall of the nasal cavity. The apex of the sinus is located near the infraorbital foramen, where the infraorbital nerve and vessels pass through. The zygomatic bone, located in the orbital floor, is in close proximity to the apex of the maxillary sinus.

**Why Each Wrong Option is Incorrect**
**Option B:** The nasal cavity is located above the maxillary sinus, and the apex of the sinus does not point towards it.
**Option C:** The orbital surface is the medial wall of the orbit, and the apex of the maxillary sinus does not point towards it.

**Clinical Pearl / High-Yield Fact**
The maxillary sinus is often involved in dental infections, particularly those involving the upper molars and premolars. This is because the roots of these teeth are in close proximity to the maxillary sinus, and infections can easily spread to the sinus.

**Correct Answer:** **✓ Correct Answer: A. Zygomatic bone**