A unilateral dilated pupil in an unconscious patient is most suggestive of uncal herniation, which occurs when the medial temporal lobe (uncus) herniates through the tentorial notch. This herniation compresses the ipsilateral third cranial nerve (oculomotor nerve), causing pupil dilation due to parasympathetic fiber disruption. The finding is a crucial surgical emergency as it indicates increased intracranial pressure with potential brainstem compression, necessitating urgent intervention to prevent irreversible damage. Other options like pontine hemorrhage typically cause pinpoint pupils, and opioid overdose usually leads to bilateral pinpoint pupils. Metabolic encephalopathy rarely causes unilateral pupillary changes.

Reference: Neurology in Clinical Practice, Bradley MD et al., 7th Edition.