**Question:** True about anatomy of adult cornea - a) Horizontal diameter is 12 mm

**Core Concept:** Anatomy of adult cornea

**Why the Correct Answer is Right:**
The correct answer is "a) Horizontal diameter is 12 mm" because this statement accurately describes the typical size of the horizontal diameter of an adult cornea. The cornea is the transparent, outermost layer of the eye that covers the corneal limbus and sclera, and it plays a crucial role in focusing light onto the retina for clear vision. An average adult cornea has a horizontal diameter of around 12 mm, making it the most prominent feature of the cornea.

**Why Each Wrong Option is Incorrect:**

- **b) Horizontal diameter is 10 mm**: This is incorrect because the average horizontal diameter of an adult cornea is approximately 12 mm, not 10 mm.
- **c) In megalocornea diameter is >12 mm**: Although the term "megalocornea" refers to an abnormally large cornea, the diameter should be greater than 12 mm, not exactly 12 mm. Thus, this option is incorrect as it does not match the average size.
- **d) In microcornea diameter is < 12 mm**: Microcornea is defined as an abnormally small cornea, and the diameter should be less than 12 mm. Therefore, this option is incorrect because it does not match the average size. **Clinical Pearls:** The diameter of the cornea can be affected by certain diseases or conditions, such as keratoconus, which is a progressive disorder of the cornea resulting in a thinning and weakening of the cornea, causing it to bulge into a cone-like shape. This can lead to the diameter of the cornea being smaller than 12 mm. Additionally, the corneal diameter may be larger than 12 mm due to conditions like ectasia or corneal dystrophy, which can cause an enlarged cornea. **Correct Answer: a) Horizontal diameter is 12 mm** In summary, the horizontal diameter of an adult cornea is typically 12 mm, although it can be larger or smaller due to various pathologies affecting the cornea. Understanding these variations is essential for diagnosing and managing ocular conditions involving the cornea.