Final answer:
Left CN II impairment is excluded when light causes only the opposite pupil to constrict, indicating damage to the oculomotor nerve. Optic nerve impairment is apparent when light fails to change pupillary size in one eye but works normally in the other. The optic nerve provides sensory input, while the oculomotor nerve facilitates the motor response in pupillary reflexes.
Step-by-step explanation:
Left CN II impairment is ruled out in the pupillary reflex question because when light is shined in the right eye and only the left pupil constricts, it indicates an intact consensual reflex but a lost direct reflex, suggesting damage to the right oculomotor nerve or Edinger-Westphal nucleus, rather than the left optic nerve. What prevents both eyes from working properly in optic nerve impairment is that when light induces no pupillary change in one eye but a normal bilateral response when shined in the other eye, it indicates damage to the optic nerve on the side where the light elicits no response. Essentially, the problem with the optic nerve prevents the sensory input necessary for initiating the pupillary reflex.
Damage to the oculomotor connections, as seen when light shined in one eye causes only the same-side pupil to constrict, would result in a consensual reflex loss, where the opposite pupil does not respond. The pupillary reflexes involve the optic nerve as the afferent limb (sensory input) and the oculomotor nerve as the efferent limb (motor response) responsible for pupil constriction or dilation.