On pupillary light reflex testing, the LEFT pupil constricts briskly to light shone in either eye, but the RIGHT pupil fails to constrict to light shone in either eye. There is no relative afferent pupillary defect. Where is the lesion?

• A Right optic nerve
• B Left optic nerve
• C Right oculomotor nerve ✓
• D Left Edinger-Westphal nucleus

Explanation

The right pupil fails to constrict to both direct (right-eye) and consensual (left-eye) light while the left pupil constricts to both, which localises the defect to the right efferent limb — the right oculomotor nerve carrying parasympathetic fibres — with the afferent pathway intact. A right optic nerve (afferent) lesion is excluded because light in the left eye would still drive consensual constriction of the right pupil (it does not), and an afferent lesion would produce a relative afferent pupillary defect. Intact left-pupil responses to light in either eye exclude a left-sided efferent or Edinger-Westphal lesion.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.