**Core Concept**
Uncal herniation is a type of transtentorial herniation where the uncus of the temporal lobe is displaced through the tentorial notch, often due to increased intracranial pressure. This can lead to compression of adjacent cranial nerves and brainstem structures.

**Why the Correct Answer is Right**
The oculomotor nerve (cranial nerve III) is the most commonly affected cranial nerve in uncal herniation due to its close proximity to the tentorial notch. The oculomotor nerve is responsible for controlling eye movements, including adduction, depression, and constriction of the pupil. Compression of the oculomotor nerve can result in ipsilateral ptosis, mydriasis, and impaired eye movements.

**Why Each Wrong Option is Incorrect**
**Option A:** The trochlear nerve (cranial nerve IV) is involved in superior oblique muscle function and is less likely to be affected in uncal herniation.
**Option B:** The trigeminal nerve (cranial nerve V) is responsible for sensory and motor functions of the face and is not typically compressed in uncal herniation.
**Option C:** The abducens nerve (cranial nerve VI) is involved in lateral rectus muscle function and can be affected in other types of brainstem herniation, but is not typically the primary cranial nerve affected in uncal herniation.

**Clinical Pearl / High-Yield Fact**
Uncal herniation can be a medical emergency and requires prompt recognition and treatment to prevent further neurological deterioration. The triad of ipsilateral ptosis, mydriasis, and impaired eye movements is a classic sign of oculomotor nerve compression.

**Correct Answer:** C.