**Core Concept**
The "tear drop" sign on X-ray of the paranasal sinuses (PNS) refers to a radiographic finding where a rounded, drop-shaped opacity is seen in the soft tissues of the maxillary sinus, typically resulting from a fracture of the orbital floor. This sign is a critical indicator of a blow out fracture of the orbit.

**Why the Correct Answer is Right**
A blow out fracture of the floor of the orbit occurs when there is a sudden increase in intraorbital pressure, often due to a direct trauma to the globe or the orbital rim. This increased pressure forces the orbital contents, including the orbital fat and the inferior rectus muscle, to herniate through the fractured orbital floor and into the maxillary sinus. As a result, a rounded, drop-shaped opacity is seen on the X-ray, which is the characteristic "tear drop" sign. This sign is a result of the herniated orbital contents displacing the normal maxillary sinus mucosa and fat.

**Why Each Wrong Option is Incorrect**
**Option A:** Comminuted fracture of the maxilla is a type of fracture where the maxilla is broken into multiple fragments. While it can cause displacement of the maxillary sinus contents, it does not typically result in the characteristic "tear drop" sign.
**Option B:** Fracture of the nasal bone is a common injury in facial trauma but does not directly affect the maxillary sinus or the orbital floor.
**Option C:** Fracture of the nasal bone may cause displacement of the nasal septum or the nasal cartilage but is not associated with the "tear drop" sign on X-ray of the PNS.
**Option D:** Fracture of the mandible is a fracture of the lower jawbone and is not related to the maxillary sinus or the orbital floor.

**Clinical Pearl / High-Yield Fact**
The "tear drop" sign on X-ray of the PNS is a critical indicator of a blow out fracture of the orbit, which can lead to complications such as diplopia, enophthalmos, and limited eye movement. A high index of suspicion is necessary to diagnose this condition, and prompt treatment is essential to prevent long-term sequelae.

✓ Correct Answer: **A.** Blow out fracture of the floor of the orbit