**Core Concept**
The question is testing the understanding of postoperative complications following cataract surgery, specifically the most common cause of vision loss in such cases. **Posterior capsular opacification (PCO)** is a common complication that can occur after cataract surgery, leading to decreased vision.

**Why the Correct Answer is Right**
Posterior capsular opacification occurs when the posterior capsule, which is the clear membrane behind the intraocular lens (IOL), becomes cloudy or opacified. This can happen due to the proliferation of lens epithelial cells and fibrosis. The posterior capsule opacifies gradually over time, leading to decreased vision. In this case, the patient's history of cataract surgery and the recent onset of diminished vision, along with the slit-lamp findings, suggest PCO as the most likely cause.

**Why Each Wrong Option is Incorrect**
* **Option A:** **Cystoid macular edema (CME)** is another potential complication of cataract surgery, but it typically presents with a more acute onset of vision loss and is often associated with inflammation. The patient's history and slit-lamp findings do not strongly suggest CME.
* **Option B:** **Endophthalmitis** is a severe and rare complication of cataract surgery, typically presenting with severe pain, redness, and vision loss. The patient's symptoms do not match this diagnosis.
* **Option C:** **Intraocular lens (IOL) dislocation** is a rare complication that can occur due to trauma or other factors. The patient's history and slit-lamp findings do not suggest IOL dislocation.

**Clinical Pearl / High-Yield Fact**
Posterior capsular opacification can often be treated with a YAG laser capsulotomy, which creates a hole in the cloudy posterior capsule, restoring clear vision.

**Correct Answer:** C.