**Core Concept**
Aniseikonia is a binocular vision disorder characterized by a significant difference in the perceived size of images formed by the two eyes. This discrepancy can lead to difficulties in depth perception, eye strain, and discomfort during binocular activities. Aniseikonia can result from various factors, including anatomical differences, refractive errors, or post-surgical complications.

**Why the Correct Answer is Right**
The correct answer, D. Difference in the size of image formed by the two eyes, accurately describes the underlying cause of aniseikonia. This condition arises when the images projected onto the retina of each eye differ significantly in size, often due to anisometropia (unequal refractive errors) or other factors that affect the optical quality of the images. The brain struggles to fuse these disparate images, leading to aniseikonia.

**Why Each Wrong Option is Incorrect**
**Option A:** Difference in the axial length of the eyeballs is a characteristic of anisometropia but does not directly cause aniseikonia. While axial length differences can contribute to the condition, aniseikonia is specifically related to the perceived size of images, not the physical dimensions of the eyeballs.

**Option B:** Difference in the size of corneas might contribute to anisometropia but is not the primary cause of aniseikonia. Corneal size differences can lead to unequal refractive errors, but the perceived image size discrepancy is the key factor in aniseikonia.

**Option C:** Difference in the size of pupils can affect light entry and potentially contribute to anisometropia, but it is not directly responsible for aniseikonia. Pupil size differences can influence image brightness, but the issue in aniseikonia lies with the perceived image size, not brightness.

**Clinical Pearl / High-Yield Fact**
A crucial aspect of managing aniseikonia is to address the underlying cause, such as anisometropia, through refractive correction (e.g., glasses or contact lenses). Additionally, aniseikonia can be a complication of refractive surgery, highlighting the importance of careful patient selection and post-operative management.

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