**Core Concept**
Regular astigmatism is a type of refractive error caused by an irregular curvature of the cornea or lens, resulting in blurred vision at all distances. This occurs due to an asymmetrical distribution of refractive power across the cornea or lens, leading to distorted images.

**Why the Correct Answer is Right**
In regular astigmatism, the corneal or lens curvature is symmetrical about the vertical or horizontal meridian, resulting in clear vision at the axis perpendicular to the astigmatic axis. This is because the cornea or lens has a regular, non-rotational astigmatism, allowing for a single axis of symmetry. The irregular curvature of the cornea or lens causes light rays to be focused at two different points, rather than a single point, leading to blurred vision at all distances.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because regular astigmatism is characterized by a symmetrical curvature of the cornea or lens, not an asymmetrical one.
**Option B:** This option is incorrect because irregular astigmatism is a different type of refractive error, characterized by an asymmetrical curvature of the cornea or lens.
**Option C:** This option is incorrect because regular astigmatism is not caused by a problem with the lens or retina, but rather by an irregular curvature of the cornea.

**Clinical Pearl / High-Yield Fact**
Regular astigmatism can be corrected with glasses, contact lenses, or refractive surgery, such as LASIK. It is essential to determine the axis of astigmatism to prescribe the correct corrective lenses.

**Correct Answer: B. regular astigmatism is characterized by an asymmetrical curvature of the cornea or lens.**