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
[{'content': 'Neutral density filter - \nSelect and explain. \nA. Amblyopia, B. Malingering, C. Afferent pupillary defect, D. Efferent pupillary defect', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Ans. (c) Afferent pupillary defectRef: Essentials of ophthalmology p. 53"Neutral density filters can be used to grade the severity of afferent pupillary defect"--Essentials of ophthalmology', 'role': 'assistant'}]