This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Ophthalmology
[{'content': 'First sign of optic nerve disease is- \nSelect and explain. \nA. Papilloedema, B. Colour blindness, C. Afferent pupillary defect, D. Efferent pupillary defect', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Marcuss Gunn pupil or RAPD (relative afferent papillary defect) is the earliest indication of optic nerve disease even in presence of normal visual acuity.', 'role': 'assistant'}]