**Core Concept**
Myopia, also known as nearsightedness, is a refractive error caused by an elongation of the eyeball, leading to an increased focal length and difficulty in focusing on distant objects. This condition results in blurred vision at a distance, while near vision remains relatively clear.

**Why the Correct Answer is Right**
Myopia is primarily caused by an axial elongation of the eyeball, which leads to a thinning of the cornea and a posterior displacement of the lens. This elongation can be attributed to genetic factors, increased near work, and a lack of outdoor activities. The eyeball's axial length increases, causing the retina to be positioned further back than normal, resulting in light being focused in front of the retina instead of directly on it.

**Why Each Wrong Option is Incorrect**
**Option A:** Myopia is primarily caused by a thickening of the cornea. This is incorrect because the primary cause of myopia is the elongation of the eyeball, not a thickening of the cornea.

**Option B:** Myopia is a result of a decrease in the refractive power of the lens. This is incorrect because myopia is actually caused by an increase in the axial length of the eyeball, which increases the focal length, not a decrease in the refractive power of the lens.

**Option C:** Myopia is a result of a posterior displacement of the lens. This is incorrect because the primary cause of myopia is the elongation of the eyeball, which leads to a posterior displacement of the lens, not the other way around.

**Clinical Pearl / High-Yield Fact**
A family history of myopia is a significant risk factor for developing the condition, and the risk increases with the number of myopic family members.

**Correct Answer:** A.