Which one of the cranial nerves exits the brainstem from the posterior aspect?
**Core Concept**
The cranial nerves arise from the brainstem, which is divided into the midbrain, pons, and medulla oblongata. Each cranial nerve has a specific origin and exits the brainstem through distinct routes, which can be either anterior or posterior.
**Why the Correct Answer is Right**
The correct answer involves the cranial nerves that exit the brainstem from the posterior aspect, specifically the accessory nerve (cranial nerve XI) and the vagus nerve (cranial nerve X), but the accessory nerve is unique because it has a cranial root that arises from the medulla oblongata and a spinal root that arises from the cervical spinal cord. However, the vagus nerve and the accessory nerve cranial root are the ones exiting the posterior aspect. The vagus nerve (cranial nerve X) exits the brainstem through the jugular foramen, while the accessory nerve (cranial nerve XI) has a cranial root that exits through the jugular foramen and a spinal root that exits through the foramen magnum.
**Why Each Wrong Option is Incorrect**
**Option A:** The trigeminal nerve (cranial nerve V) exits the brainstem from the anterior aspect through the superior orbital fissure and foramen rotundum, making it an incorrect choice.
**Option B:** The facial nerve (cranial nerve VII) exits the brainstem from the anterior aspect through the internal auditory meatus, which makes it an incorrect option.
**Option C:** The glossopharyngeal nerve (cranial nerve IX) exits the brainstem from the anterior aspect through the jugular foramen, making it an incorrect choice.
**Clinical Pearl / High-Yield Fact**
Remember that the accessory nerve (cranial nerve XI) has a unique origin with both a cranial and spinal root, and its cranial root exits the brainstem from the posterior aspect through the jugular foramen.
**Correct Answer: C. The accessory nerve (cranial nerve XI) has a cranial root that exits the posterior aspect of the brainstem.**