An afferent pupillary defect can occur only if the lesion is anterior to the ______.

• A. LGN
• B. Optic chiasm
• C. Optic tract
• D. Visual cortex

Answer: (A)

Afferent pupillary defect reflects a loss of the retinal/fibers carrying the light signal before it reaches the midbrain’s pupillary reflex circuitry. The retina sends the afferent signal to the pretectal area via the retino-pretectal projection, and this input triggers bilateral EW nucleus activity to constrict the pupils. This reflex pathway is disrupted only when the lesion lies before the lateral geniculate nucleus, i.e., anterior to the LGN. If the lesion is at or behind the LGN (posterior to it), the afferent input to the pretectal area can still reach the reflex arc, so the pupil light response may remain intact despite visual perception loss. Therefore the boundary is the lateral geniculate nucleus.