Which of the following cranial nerve is a pure motor nerve?

**Core Concept**
The cranial nerves are a group of 12 nerves that arise directly from the brain and are responsible for controlling various functions such as sensation, movement, and autonomic functions. Among the cranial nerves, some are purely motor, while others are purely sensory or have both motor and sensory functions.

**Why the Correct Answer is Right**
The cranial nerve that is a pure motor nerve is the **oculomotor nerve (CN III)**. This nerve is responsible for controlling the movement of the eyeball, including the constriction of the pupil and the movement of the eyelid. It is a mixed nerve, but it has no sensory fibers. The oculomotor nerve originates from the midbrain and synapses in the superior colliculus before innervating the extraocular muscles. The oculomotor nerve is the thickest of all the cranial nerves and has three branches: the superior, inferior, and nasociliary branches.

**Why Each Wrong Option is Incorrect**
* **Option A:** The trochlear nerve (CN IV) is a pure motor nerve, but it is not the correct answer because it is responsible for controlling the superior oblique muscle of the eye, which is a distinct function from the oculomotor nerve.
* **Option B:** The trigeminal nerve (CN V) is a mixed nerve with both motor and sensory functions, making it an incorrect choice.
* **Option D:** The facial nerve (CN VII) is also a mixed nerve with both motor and sensory functions, particularly responsible for controlling the muscles of facial expression.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the oculomotor nerve (CN III) is responsible for controlling the movement of the eyeball, including the constriction of the pupil and the movement of the eyelid. Damage to this nerve can result in ptosis (drooping of the eyelid) and diplopia (double vision).

**Correct Answer:** C. The oculomotor nerve (CN III) is a pure motor nerve.