## Core Concept
Microcornea is a rare congenital anomaly characterized by an abnormally small cornea. It is often associated with other ocular and systemic abnormalities. The definition of microcornea is crucial for ophthalmologists to diagnose and manage the condition properly.

## Why the Correct Answer is Right
The correct answer defines microcornea as a cornea with a diameter less than 10 mm in infants or less than 11 mm in adults. This condition can lead to various vision problems and may be associated with glaucoma, cataracts, and other ocular anomalies. The corneal diameter is a critical factor in diagnosing microcornea.

## Why Each Wrong Option is Incorrect
* **Option A:** This option is incorrect because it does not provide a specific definition for microcornea. Without a clear corneal diameter measurement, it's impossible to accurately diagnose the condition.
* **Option B:** This option is incorrect because it provides a definition that is not specific to microcornea or is not accurate. The correct definition involves specific corneal diameter measurements.
* **Option C:** This option is incorrect because, similar to Option B, it does not accurately define microcornea or provides a measurement that does not align with established ophthalmic criteria.

## Clinical Pearl / High-Yield Fact
A key point to remember is that microcornea is often associated with other ocular anomalies and systemic conditions. Early diagnosis and management are crucial to prevent complications such as glaucoma and vision loss. A corneal diameter of less than 10 mm in infants and less than 11 mm in adults is a critical threshold for diagnosing microcornea.

**Correct Answer: D. Corneal diameter less than 10 mm in infants or less than 11 mm in adults.**