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Background: Relative afferent pupillary defects are typically related to ipsilateral lesions within the anterior visual pathways. Objective: To describe a patient who had a workup for headache and was found to have an isolated left relative afferent pupillary defect without any other neurological findings. Dosign: We review the neuroanatomy of the pupillary light reflex pathway and emphasize the nasotemporal bias of decussating fiber projections, which accounts for the relative afferent pupillary defect contralateral to the described lesion. Result: Magnetic resonance imaging of the brain revealed a pineal tumor compressing the right rostral midbrain. Conclusion: While rare, a relative afferent pupillary defect can occasionally occur secondary to lesions in the postchiasmal pathways. In these circumstances, the pupillary defect will be observed to be contralateral to the side of the lesion. [ABSTRACT FROM AUTHOR]