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
Astigmatism is considered to be: 
Select and explain. 
A. Spherical abberation, B. Curvatural ametropia, C. Axial ametropia, D. Index ametropia
A: Spherical abberation
Answer: A: Spherical abberation 
Explanation: Ans. Spherical abberation