**Core Concept**
Astigmatism is a type of refractive error caused by an irregular curvature of the cornea or lens inside the eye, leading to distorted vision. This distortion occurs because light rays are not focused equally in all meridians, resulting in blurry or hazy vision.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of astigmatism. Astigmatism occurs when the cornea or lens has an irregular curvature, causing light to be focused at two different points instead of one. This is due to the unequal distribution of corneal or lenticular curvature in different meridians. The result is that light rays are not focused equally, leading to distorted vision. In some cases, astigmatism can be caused by a corneal or lenticular scar, injury, or disease.

**Why Each Wrong Option is Incorrect**

**Option A:** Incorrect because myopia (nearsightedness) is a type of refractive error, but it is not directly related to the irregular curvature of the cornea or lens.

**Option B:** Incorrect because hyperopia (farsightedness) is a type of refractive error, but it is not caused by an irregular curvature of the cornea or lens.

**Option C:** Incorrect because presbyopia is a type of age-related refractive error that typically affects individuals over the age of 40. It is not directly related to the irregular curvature of the cornea or lens.

**Clinical Pearl / High-Yield Fact**
Astigmatism can be corrected with glasses, contact lenses, or refractive surgery. It is essential to diagnose and treat astigmatism promptly to prevent vision loss and other complications.

**Correct Answer: C. Irregular curvature of the cornea or lens.**