This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Optics and refraction, Subject name: Ophthalmology
Astigmatism is due to - 
Select and explain. 
A. Irregularity of curvature of cornea, B. Irregularity of curvature of lens, C. Forward displacement of the lens, D. Backward displacement of lens
A: Irregularity of curvature of cornea
Answer: A: Irregularity of curvature of cornea 
Explanation: Astigmatism is a type of refractive error wherein the refraction varies in the different meridia. Consequently, the rays of light entering in the eye cannot converge to a point focus but form focal lines. Broadly, there are two types of astigmatism: regular and irregular Corneal astigmatism is the result of abnormalities of curvature of cornea. It constitutes the most common cause of astigmatism Reference : A K KHURANA OPHTALMOLOGY,Edition 4,Page-34