## **Core Concept**
The patient's symptoms suggest a problem with the afferent pupillary pathway, which is responsible for transmitting visual information from the retina to the brain, specifically to the midbrain, where it controls pupillary constriction. The afferent pathway involves the retina, optic nerve, and optic tract, while the efferent pathway involves the Edinger-Westphal nucleus, oculomotor nerve (cranial nerve III), and the ciliary ganglion.

## **Why the Correct Answer is Right**
The patient's presentation indicates a unilateral afferent pupillary defect (Marcus Gunn pupil). When light is shone in the right eye, there is no pupillary response in either eye, indicating that the afferent signal from the right eye is not being properly transmitted. However, when light is shone in the left eye, both eyes constrict, indicating that the efferent pathway is intact. This pattern suggests a lesion affecting the afferent pathway from the right eye. The most likely location for such a lesion is in the **optic nerve** of the right eye, as lesions in the optic tract would typically cause a contralateral homonymous hemianopia and a relative afferent pupillary defect.

## **Why Each Wrong Option is Incorrect**
- **Option A:** A lesion in the **optic tract** would cause a contralateral homonymous hemianopia and typically a relative afferent pupillary defect, not a complete absence of pupillary response on the affected side.
- **Option B:** A lesion in the **Edinger-Westphal nucleus** or **oculomotor nerve (CN III)** would affect the efferent pathway, leading to an inability to constrict the pupil (pupillary dilation), which is not the case here since the efferent pathway appears intact (as evidenced by the pupillary response to light in the left eye).
- **Option D:** A lesion in the **midbrain** could affect both afferent and efferent pupillary pathways and might cause more complex pupillary reactions, including fixed pupils, which does not match this patient's presentation.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that an afferent pupillary defect (Marcus Gunn pupil) points towards a lesion in the **optic nerve** or severe retinal disease. This finding is critical in the neuro-ophthalmic examination as it helps localize the lesion.

## **Correct Answer: C. Optic nerve.**