Final answer:
The parasympathetic fibers are affected first by compression of the oculomotor (CN III) nerve.
Step-by-step explanation:
The cranial nerve III, also known as the oculomotor nerve, contains both motor and parasympathetic fibers. However, when it comes to compression, the parasympathetic fibers are affected first.
These fibers control the pupillary size by constricting the iris muscles. When the oculomotor nerve is compressed, it can result in an uneven pupil size or a non-reactive pupil.
On the other hand, the motor fibers of the oculomotor nerve control the movement of the eye muscles. Compression of these fibers can lead to eye movement abnormalities or weakness in certain eye muscles.