Posterior communicating artery aneurysm
PCOM. I kept forgetting this important to look for diagnosis. I have been reading this multiple times .
If the pupil is large on one side of the eye, Isolated cranial nerve III palsy
the parasympathetic fibres of the cranial nerves III are arranged on the outer surface as compared to the motor fibers. Therefore an external compression would cause dilated pupil but not in the case of medical cranial nerve III due to mononeuritis.