This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Neuro-ophthalmology, Subject name: Ophthalmology
Marcus Gunn pupil is a feature of 
Select and explain. 
A. Ciliary ganglion lesion, B. Lesion of Edinger-Westphal nucleus, C. Optic neuritis, D. Papilloedema
C: Optic neuritis
Answer: C: Optic neuritis 
Explanation: OPTIC NEURITIS Signs: 1. Visual acuity is usually reduced markedly. 2. Colour vision is often severely impaired. 3. Pupil shows ill-sustained constriction to light. Marcus Gunn pupil which indicates relative afferent pupillary defect (RAPD) is a diagnostic sign. It is detected by the swinging flash light test. Swinging flash light test: It is performed when relative afferent pathway defect is suspected in one eye (unilateral optic nerve lesion with good vision). To perform this test, a bright flash light is shone on to one pupil and constriction is noted. Then the flash light is quickly moved to the contralateral pupil and response noted. This swinging to-and-fro of flash light is repeated several times while observing the pupillary response. Normally, both pupils constrict equally and the pupil to which light is transferred remains tightly constricted. In the presence of relative afferent pathway defect in one eye, the affected pupil will dilate when the flash light is moved from the normal eye to the abnormal eye. This response is called 'Marcus Gunn pupil' or a relative afferent pupillary defect (RAPD). It is the earliest indication of optic nerve disease even in the presence of normal visual acuity. Ref:- A K KHURANA; pg num:-294,474,475