###Rationale: Ans: d (Oblique) Ref: AK Khurana, ed, p. 60-61; 4th p. 36Astigmatism: A type of refractive error in which refraction varies in different meridian so that light entering the eye cannot converge to a point focus but form focal lines.Regular astigmatism: Refractive index changes uniformly: 2 principal mediaIrregular astigmatism: Irregular change of refractive index in different meridian multiple mediaAccording to etiologyTypes of regular astigmatism(depending on axis & angle between two principal meridian)1. Comeal astigmatism2. Lenticular astigmatisma. Curvaturalb. Posturalc. Index3. Retinal astigmatism1. With the rule astigmatism: Two principal meridian right angle to one another but more curved than vertical2. Against the rule: Horizontal media more curved than vertical3. Oblique astigmatism: Two principal media are not horizontal or vertical though these are at right angles to each other4. Bioblique astigmatism: Two principle meridian are not vertical or horizontal and also not perpendicular to each other.

###Answer: OPTION D IS CORRECT.
###Question: When the largest diameter and shortest diameter are perpendicular to each other the type o astigmatism is:
###Options:
A. Regular
B. Irregular
C. Lenticular
D. Oblique
