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': 'The most characteristic visual field change in primary open-angle glaucoma is: \nSelect and explain. \nA. Nerve fibre bundle defect, B. Enlargement of blind spot, C. Generalised constriction of field, D. Sector-shaped defects', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ans. Nerve fibre bundle defect', 'role': 'assistant'}]