**Core Concept**
Amblyopia, also known as lazy eye, is a vision development disorder that affects the brain's ability to properly connect with one or both eyes. It is a result of abnormal visual experience during critical periods of visual development, leading to permanent vision loss if left untreated.

**Why the Correct Answer is Right**
The correct answer lies in the concept of binocular vision and the brain's ability to process visual information from both eyes. In amblyopia, the brain fails to develop normal binocular vision, resulting in reduced vision in one eye. This defect is due to a lack of normal visual experience during critical periods of visual development, which can be caused by factors such as strabismus (crossed eyes), anisometropia (unequal refractive errors), or cataracts.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because amblyopia is not a result of a defect in the eye itself, but rather a defect in the brain's ability to process visual information.
**Option B:** This option is incorrect because strabismus is a potential cause of amblyopia, but it is not the defect that lies at the heart of the condition.
**Option C:** This option is incorrect because while anisometropia can be a cause of amblyopia, it is not the underlying defect that causes the condition.

**Clinical Pearl / High-Yield Fact**
A key aspect of amblyopia is that it can only be treated effectively during critical periods of visual development, typically before the age of 7. If left untreated, amblyopia can lead to permanent vision loss.

**Correct Answer: D. Defect in the brain's ability to process visual information.