What cranial nerve is involved when there is no consensual or direct reaction in the affected eye while the unaffected eye reacts normally?

The scenario described pertains to an issue involving the pupillary light reflex, which involves the sensory and motor pathways mediated by specific cranial nerves. In this reflex, light shone into one eye causes both pupils to constrict; this is known as the consensual reflex.

When light is directed into one eye and there is no response in the affected eye (the direct reaction), while the unaffected eye constricts normally, it indicates that there is a problem with the motor aspect of the pupillary reflex in the affected eye. This motor response is primarily governed by the oculomotor nerve, also known as cranial nerve III.

Cranial nerve III is responsible for controlling the majority of the eye's movements and is crucial in mediating the constriction of the pupil through its parasympathetic fibers. If there is damage to this nerve, it can lead to an inability to constrict the pupil in the affected eye despite the unaffected eye reacting normally.

The other cranial nerves listed do not directly contribute to the responses of the pupillary light reflex in the manner described. Cranial nerve VI is primarily responsible for lateral eye movement, CN II is involved in transmitting visual information, and CN IV controls the superior oblique muscle of