**Core Concept**
Lens implant surgery, also known as cataract surgery, involves removing the cloudy lens of the eye and replacing it with an artificial lens implant. The procedure is commonly performed to restore vision in individuals with cataracts. The most frequent complication of this surgery is related to the posterior capsule of the lens.

**Why the Correct Answer is Right**
The commonest side effect of lens implant surgery is posterior capsule opacification (PCO). This occurs when the posterior capsule, which is left intact during the surgery, becomes cloudy or opacified over time. PCO is caused by the proliferation of residual lens epithelial cells and fibrosis within the posterior capsule. The opacification can lead to a decrease in visual acuity and may require a secondary surgical procedure to clear the capsule.

**Why Each Wrong Option is Incorrect**
**Option A:** Uveitis is an inflammation of the uvea, the pigmented layer of the eye, and is not a common side effect of lens implant surgery.
**Option B:** Retinal detachment is a serious complication of cataract surgery, but it is not the most frequent side effect.
**Option C:** Endophthalmitis is a rare but serious infection of the interior of the eye, and is not the most common side effect of lens implant surgery.

**Clinical Pearl / High-Yield Fact**
The risk of posterior capsule opacification can be reduced by the use of a capsulotomy, a procedure where the posterior capsule is opened with a laser, during the initial cataract surgery.

**Correct Answer: D. Posterior capsule opacification.**