**Core Concept**
Irregular astigmatism is a type of astigmatism characterized by non-uniform curvature of the cornea or lens, resulting in irregularly shaped principal meridians. This condition leads to distorted vision, often with multiple axes of astigmatism. The irregular curvature of the cornea or lens causes light to be focused at two or more points, rather than a single point, resulting in blurry or distorted vision.

**Why the Correct Answer is Right**
In irregular astigmatism, the principal meridians are not perpendicular to each other, unlike in regular astigmatism where the principal meridians are perpendicular. This non-perpendicularity of principal meridians is due to the irregular curvature of the cornea or lens, which deviates from the normal cylindrical shape. The irregular astigmatism can be caused by various factors, including trauma, keratoconus, or ectatic corneal disorders. The non-perpendicular principal meridians in irregular astigmatism make it challenging to correct with standard astigmatism correction methods, such as glasses or contact lenses.

**Why Each Wrong Option is Incorrect**
**Option A:** Perpendicular principal meridians are characteristic of regular astigmatism, not irregular astigmatism. In regular astigmatism, the principal meridians are perpendicular to each other, and the curvature of the cornea or lens is uniform.

**Option C:** This option is incorrect because irregular astigmatism is specifically defined by non-perpendicular principal meridians, not any of the above. Regular astigmatism, with perpendicular principal meridians, is a different condition.

**Option D:** This option is incorrect because non-perpendicular principal meridians are indeed a characteristic of irregular astigmatism, making option B the correct answer.

**Clinical Pearl / High-Yield Fact**
Irregular astigmatism can be challenging to diagnose and correct, and it's essential for ophthalmologists to recognize the non-perpendicular principal meridians as a key characteristic of this condition. A thorough examination, including keratometry and corneal topography, is necessary to diagnose irregular astigmatism and determine the best course of treatment.

**✓ Correct Answer: B. Non perpendicular principal meridians**