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
Thinning of cornea occurs in: 
Select and explain. 
A. Megalocornea, B. Bullous keratopathy, C. Endothelial dystrophy, D. Keratoconus
D: Keratoconus
Answer: D: Keratoconus 
Explanation: Ans. Keratoconus