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
Dellen is: 
Select and explain. 
A. Localized thinning of peripheral cornea, B. Raised lesions in corneo limbal junction, C. Age related macular degeneration, D. Marginal keratitis
A: Localized thinning of peripheral cornea
Answer: A: Localized thinning of peripheral cornea 
Explanation: Ans. Localized thinning of peripheral cornea