Options (b) and (c) are eliminated because isolated lesion in segment 3 alone or in segment 5 alone cannot produce the light reflex abnormalities in question. A single lesion anywhere along segment 1, the left afferent limb, which includes the left retina, left optic nerve, and left pretectal nucleus, can produce the light reflex abnormalities observed. Examples of segment 1 pathologies include left optic neuritis (inflammation or infection of the left optic nerve), detachment of left retina, and an isolated small stroke involving only the left pretectal nucleus.