**Core Concept**
CSF rhinorrhea refers to the leakage of cerebrospinal fluid (CSF) through the nose due to a defect in the dura mater or other structures of the nasal cavity. This condition can be caused by trauma, surgery, or congenital defects.

**Why the Correct Answer is Right**
The most common site of CSF leak in rhinorrhea is the cribriform plate, a thin, porous bone structure in the anterior cranial fossa that forms the roof of the nasal cavity. The cribriform plate is a weak point in the skull base due to its thinness and porous nature, making it susceptible to fractures and subsequent CSF leaks. The foramina in the cribriform plate provide a pathway for olfactory nerve fibers to pass from the nasal cavity to the brain, but they also create a potential route for CSF to escape.

**Why Each Wrong Option is Incorrect**
**Option A:** The ethmoid sinus is a potential site of CSF leak, but it is not the most common site. CSF leaks from the ethmoid sinus are often associated with more extensive fractures or defects.
**Option B:** The frontal sinus is another potential site of CSF leak, but it is less common than the cribriform plate.
**Option C:** The sphenoid sinus is a rare site of CSF leak, and it is often associated with more complex skull base fractures.

**Clinical Pearl / High-Yield Fact**
The cribriform plate is a critical area in the diagnosis and management of CSF rhinorrhea, and a high index of suspicion is needed to identify patients with this condition.

**Correct Answer:** C. Cribriform plate.