Astigmatism is due to –
**Core Concept**
Astigmatism is a type of refractive error caused by an irregular shape of the cornea or lens in the eye, leading to blurred vision at all distances. This condition is characterized by an asymmetrical curvature of the cornea or lens, which prevents light from focusing properly on the retina.
**Why the Correct Answer is Right**
The correct answer is due to an irregular curvature of the cornea or lens, which affects the shape of the optical surface. In astigmatism, the curvature of the cornea or lens is not symmetrical, resulting in multiple focal points rather than a single focal point. This is often due to an oblique astigmatism, where the curvature is more pronounced in one meridian than the other.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because irregular curvature of the cornea or lens is the primary cause of astigmatism, not a defect in the crystalline lens.
* **Option B:** This option is incorrect because presbyopia is a age-related condition that affects the ability to focus on near objects, but it is not related to astigmatism.
* **Option C:** This option is incorrect because myopia is a type of nearsightedness caused by a longer axial length of the eye, which is not related to astigmatism.
**Clinical Pearl / High-Yield Fact**
Astigmatism can be corrected with glasses, contact lenses, or refractive surgery, such as LASIK.
**Correct Answer:** C. Myopia is not the answer as it is a different type of refractive error.