**Core Concept**
Aniseikonia is a condition characterized by a perceived difference in the size of images formed by the two eyes, leading to visual disturbances and potential eye strain. This phenomenon arises from discrepancies in the optical properties of the two eyes, affecting the way images are perceived and processed by the brain.

**Why the Correct Answer is Right**
The correct answer, D. Difference in the size of image formed by the two eyes, accurately describes aniseikonia. This condition can result from various factors, such as anisometropia (differences in refractive errors between the two eyes), strabismus (crossed eyes), or other anomalies in the shape or size of the eyes. When images from the two eyes are perceived as being different sizes, the brain struggles to fuse them, leading to visual disturbances and discomfort. This phenomenon is closely related to the concept of binocular vision, where the brain combines images from both eyes to create a single, unified visual perception.

**Why Each Wrong Option is Incorrect**
**Option A:** Difference in the axial length of the eyeballs is related to myopia (nearsightedness) or hyperopia (farsightedness), but it does not directly cause aniseikonia.

**Option B:** Difference in the size of the cornea is not a primary factor in aniseikonia. While corneal irregularities can affect vision, they do not directly result in perceived differences in image size.

**Option C:** Difference in the size of the pupil is not a primary factor in aniseikonia. Pupil size affects the amount of light entering the eye, but it does not directly influence the perceived size of images.

**Clinical Pearl / High-Yield Fact**
Aniseikonia can be a significant issue in patients with anisometropic amblyopia (lazy eye), where the brain favors one eye over the other due to differences in refractive errors. Treatment often involves corrective lenses or surgeries to align the eyes and improve binocular vision.

**✓ Correct Answer: D. Difference in the size of image formed by the two eyes**