Question

Relative afferent pupillary defect (RAPD) is characteristically seen in damage to:

• A. Optic nerve
• B. Optic tract
• C. Lateral geniculate body
• D. Oculomotor nerve

Hint:

RAPD is an input-side defect found with the swinging flashlight test; which structure carries that afferent signal?

Answer 1

The Correct Option is A

To answer this, recall how the pupillary light reflex is wired. Light entering an eye is sensed by the retina and sent up the optic nerve; the signal then crosses over so that shining a light in either eye normally makes both pupils shrink by the same amount. This sensing pathway is the afferent (input) side of the reflex, while the efferent (output) side travels in the oculomotor nerve to constrict the pupil. A relative afferent pupillary defect is purely an input-side problem, and it is brought out by swinging a torch back and forth between the two eyes. If one optic nerve is diseased, say from optic neuritis, that eye delivers a weaker light message. When the torch swings onto the bad eye, the brain registers less light, so instead of constricting, both pupils drift wider; that paradoxical dilation is the RAPD. Damage to the oculomotor nerve cannot cause this, because that nerve is the output cable; it produces a fixed, dilated pupil on the affected side rather than a relative afferent defect. Lesions further back, in the optic tract or lateral geniculate body, mostly create field cuts but, because of heavy fibre crossing, do not give the clean RAPD typical of optic nerve disease. The characteristic site for RAPD is therefore the optic nerve. \[\boxed{\text{Optic nerve}}\]