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': 'Thinning of cornea occurs in: \nSelect and explain. \nA. Megalocornea, B. Bullous keratopathy, C. Endothelial dystrophy, D. Keratoconus', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Ans. Keratoconus', 'role': 'assistant'}]