## Core Concept
Astigmatism is a refractive error caused by an irregular curvature of the cornea or lens inside the eye, leading to blurred or distorted vision. The type of astigmatism is determined by the orientation of its meridians (axes), specifically the largest diameter (most plus or least minus power) and the shortest diameter (most minus or least plus power).

## Why the Correct Answer is Right
When the largest diameter and shortest diameter are perpendicular to each other, it implies that the meridians of the astigmatism are at right angles (90 degrees apart). This specific orientation defines **with-the-rule astigmatism** if the more plus (or less minus) meridian is at or near 90 degrees (vertical axis) and **against-the-rule astigmatism** if it's at or near 180 degrees (horizontal axis). However, the general term for astigmatism where these meridians are perpendicular is **simple astigmatism** or more specifically in this context, **with-the-rule** or **against-the-rule** depending on axis orientation. The question seems to point towards a general understanding that when these are perpendicular, it typically refers to a common form of astigmatism without further specification.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because it does not specify the type of astigmatism related to the orientation described.
- **Option B:** This option is incorrect for similar reasons; it lacks specificity regarding the orientation of astigmatism.
- **Option C:** This seems to be the correct orientation but let's evaluate D.
- **Option D:** This option is incorrect as it does not accurately describe the astigmatism type related to perpendicular diameters.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **with-the-rule astigmatism** is more common and usually has its more positive (or less negative) meridian along the 90-degree axis (vertical), while **against-the-rule** has it along the 180-degree axis (horizontal). When these are perpendicular, it typically aligns with one of these common forms.

## Correct Answer: C.