**Core Concept**
CSF rhinorrhea is a condition characterized by the leakage of cerebrospinal fluid (CSF) through the nose, resulting from a defect in the dura mater, a protective membrane covering the brain and spinal cord. This condition can lead to complications such as meningitis and encephalitis if left untreated.

**Why the Correct Answer is Right**
The correct answer is related to the anatomical structure that separates the cranial cavity from the nasal cavity. The cribriform plate, a thin and porous bone, forms part of the anterior cranial fossa and is a common site for CSF leaks. The dura mater is attached to the cribriform plate, and any defects or fractures in this area can lead to CSF rhinorrhea.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the ethmoid bone is a separate structure that forms part of the nasal cavity, but it is not directly related to the leakage of CSF.
**Option B:** This option is incorrect because the frontal sinus is a paranasal sinus that drains into the nasal cavity, but it is not a primary site for CSF leaks.
**Option C:** This option is incorrect because the sphenoid sinus is another paranasal sinus that drains into the nasal cavity, but it is not directly related to the leakage of CSF.

**Clinical Pearl / High-Yield Fact**
It is essential to note that CSF rhinorrhea can be a complication of head trauma, surgery, or other conditions that result in a defect in the dura mater. A high index of suspicion is required to diagnose this condition, and imaging studies such as CT or MRI scans may be necessary to confirm the diagnosis.

**Correct Answer:** C.