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
First sign of optic nerve disease is: 
Select and explain. 
A. Papilloedema, B. Colour blindness, C. Afferent pupillary defect, D. Efferent pupillary defect
C: Afferent pupillary defect
Answer: C: Afferent pupillary defect 
Explanation: C i.e. Afferent pupillary defect Marcuss Gunn pupil or RAPD (relative afferent papillary defect) is the earliest indication of optic nerve disease even in presence of normal visual acuity.Q