This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Elementary Optics, 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. Irregularity of curvature of cornea > Irregularity of curvature of lens
D: Irregularity of curvature of cornea > Irregularity of curvature of lens
Answer: D: Irregularity of curvature of cornea > Irregularity of curvature of lens 
Explanation: Ans. (d)Ref: Khurana optics 2/e, p. 79; Parsons 22/e, p. 52Most common cause of astigmatism is abnormalities of curvature of cornea. It is usually congenitalLenticular astigmatism is rare, seen in lenticonus (cone shaped lens), congenital tilling, oblique placement, traumatic subluxation of lens