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
What is irregular astigmatism - 
Select and explain. 
A. Perpendicular principal meridians, B. Non perpendicular principal meridians, C. Any of the above, D. None of the above
B: Non perpendicular principal meridians
Answer: B: Non perpendicular principal meridians 
Explanation: It is characterized by an irregular change of refractive power in different meridia. There are multiple meridia which admit no geometrical analysis.the principal meridians are not perpendicular to each other Image : Corneal topographies in regular and irregular astigmatisms. A: Regular astigmatism of 4.0 diopters with two perpendicular main meridians. B: Irregular astigmatism of 6.5 diopters with non-perpendicular main meridians. Reference : A K KHURANA OPHTALMOLOGY,Edition 4,Page-38