**Core Concept**
CSF rhinorrhea is a condition characterized by the leakage of cerebrospinal fluid (CSF) from the nasal cavity, resulting from a defect in the dura mater and the bony structures that separate the cranial cavity from the nasal cavity. This condition is often associated with traumatic injuries to the skull base.

**Why the Correct Answer is Right**
The most common cause of CSF rhinorrhea is a fracture of the cribriform plate, which is a thin, porous bone that forms the roof of the nasal cavity. The cribriform plate is the weakest point in the anterior cranial fossa and is susceptible to fractures from direct trauma to the forehead or nose. When the cribriform plate is fractured, the dura mater and the CSF that lies beneath it can leak through the defect, resulting in CSF rhinorrhea.

**Why Each Wrong Option is Incorrect**
**Option A:** Fracture of the ethmoid bone, while possible, is less common than fracture of the cribriform plate as a cause of CSF rhinorrhea.
**Option B:** Fracture of the maxillary sinus can cause CSF rhinorrhea, but it is not the most common cause.
**Option C:** Fracture of the frontal sinus can also cause CSF rhinorrhea, but it is less common than fracture of the cribriform plate.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that CSF rhinorrhea can be a delayed presentation after a traumatic injury, and it may not be immediately apparent at the time of the initial trauma. Therefore, a high index of suspicion is necessary in patients with a history of trauma who present with nasal discharge or other symptoms suggestive of CSF rhinorrhea.

**Correct Answer:** C. Fracture of the cribriform plate is the most common cause of CSF rhinorrhea.