This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Diseases of Cornea, Subject name: Ophthalmology
Maximum visual impairment occurs in: 
Select and explain. 
A. Leukoma adherens at periphery, B. Centrocaecal leucoma, C. Nebular corneal opacity, D. Med peripheral Leucoma
C: Nebular corneal opacity
Answer: C: Nebular corneal opacity 
Explanation: Nebular corneal opacity has maximal visual disability. Type of corneal opacities Nebular: Superficial layers involving upto superficiial stroma. It do more disability of vision owing to the irregular astigmatism. Macular: Upto mid stroma is involved as opacity Leucomatous: Full thickness stromal opacity. Adherent leucoma is iris tissue incarcerated in opacity, seen normally after perforation. Peripheral, mid peripheral and centrocaecal leucomas will not affect the visual axis and hence would not produce visual disability. However a central leucoma would not allow light to pass through and hence would cause maximum visual problems.