Anterior ethmoidal nerve supplies all except:
**Core Concept**
The anterior ethmoidal nerve is a branch of the nasociliary nerve, which in turn arises from the ophthalmic division (V1) of the trigeminal nerve (CN V). It provides sensory innervation to the anterior part of the nasal cavity and the adjacent ethmoidal air cells.
**Why the Correct Answer is Right**
The anterior ethmoidal nerve supplies the anterior part of the nasal septum, the superior part of the nasal vestibule, and the anterior ethmoidal air cells. This is due to its anatomical location and the distribution of its branches. The nerve provides sensory innervation to these areas, transmitting sensations of touch, pressure, and pain.
**Why Each Wrong Option is Incorrect**
* **Option A:** Incorrect because the posterior ethmoidal nerve is a separate branch of the nasociliary nerve, which supplies the posterior part of the nasal cavity and the posterior ethmoidal air cells.
* **Option B:** Incorrect because the lacrimal nerve, a branch of the ophthalmic nerve, supplies the lacrimal gland and the conjunctiva, not the nasal cavity.
* **Option C:** Incorrect because the external nasal nerve, a branch of the nasal branch of the maxillary nerve, supplies the skin of the external nose, not the nasal cavity.
**Clinical Pearl / High-Yield Fact**
The anterior ethmoidal nerve is often involved in the transmission of referred pain from the nasal cavity to the forehead, particularly in the distribution of the ophthalmic nerve.
**Correct Answer: A. The posterior ethmoidal nerve.**