**Core Concept**
Anisometropia refers to a condition where there is a significant difference in the refractive power between the two eyes, often resulting in unequal vision between the two eyes. This difference in refractive power can be due to variations in the cornea, lens, or axial length of the eye.

**Why the Correct Answer is Right**
Anisometropia occurs when the two eyes have different refractive indices, which can be caused by differences in corneal curvature, lens thickness, or axial length. This condition can lead to difficulties in binocular vision, depth perception, and even amblyopia in the eye with the lower refractive power. The difference in refractive power between the two eyes can be measured using retinoscopy or autorefraction.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because heterophoria refers to a latent deviation of the eye, which is not related to the difference in refractive power between the two eyes.
**Option B:** This option is incorrect because astigmatism refers to an irregular curvature of the cornea or lens, which can cause blurred vision at all distances, but it does not necessarily result in a significant difference in refractive power between the two eyes.
**Option C:** This option is incorrect because presbyopia refers to the age-related loss of accommodation, which affects the ability of the eye to focus on near objects, but it is not related to the difference in refractive power between the two eyes.

**Clinical Pearl / High-Yield Fact**
It's essential to note that anisometropia can be caused by various factors, including genetic predisposition, trauma, or previous eye surgery. A thorough ophthalmic examination, including autorefraction and retinoscopy, is necessary to diagnose and manage anisometropia.

**Correct Answer: C. Presbyopia.**