A relative afferent pupillary defect (RAPD) is detected on the swinging flashlight test. This indicates a lesion in:

• A The efferent limb of the pupillary reflex arc (oculomotor nerve or ciliary ganglion)
• B The optic radiation or visual cortex (posterior visual pathway)
• C The afferent limb anterior to the chiasm (optic nerve or retina) with asymmetric or unilateral dysfunction ✓
• D Bilateral symmetric optic nerve lesions

Explanation

RAPD (Marcus Gunn pupil) indicates asymmetric damage to the afferent limb of the pupillary reflex — specifically the optic nerve or retina anterior to the optic chiasm on one side (or asymmetrically). When the flashlight is swung to the affected side, both pupils paradoxically dilate because the reduced afferent signal from the diseased eye produces less pupillary constriction. Posterior pathway (chiasm to cortex) lesions do not produce RAPD. Efferent lesions cause abnormal pupil reactions but not RAPD. Bilateral symmetric optic nerve lesions also do not produce RAPD.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.