With-the-rule astigmatism is correctly described as:
- A The vertical meridian of the cornea has greater refractive power, corrected by a minus cylinder with axis at 180 degrees ✓
- B The horizontal meridian of the cornea has greater refractive power, corrected by a minus cylinder with axis at 90 degrees
- C The principal meridians lie at 45 and 135 degrees
- D Both principal meridians have equal refractive power but differ in curvature
Correct answer: A. The vertical meridian of the cornea has greater refractive power, corrected by a minus cylinder with axis at 180 degrees
Explanation
In with-the-rule astigmatism the cornea is steeper in the vertical meridian, so the vertical meridian carries the greater refracting power and the correcting minus cylinder axis is horizontal at 180 degrees. This pattern is physiological and common in youth. When the horizontal meridian is steeper, as typically develops in older age from lid laxity, the astigmatism is against the rule, and meridians near 45 and 135 degrees define oblique astigmatism. Option D contradicts the definition, since unequal powers are essential to any astigmatism.
Reference: Khurana AK, Comprehensive Ophthalmology, 7th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.