### **Core Concept**
Endophthalmitis is a severe inflammatory condition of the inner coats of the eye, typically caused by an infection. It is a medical emergency that can occur after cataract surgery, often presenting with symptoms such as increasing pain, decreased vision, and redness of the eye.

### **Why the Correct Answer is Right**
Endophthalmitis is a potential complication of cataract surgery, usually caused by bacterial, fungal, or viral infections. The most common organisms responsible are Staphylococcus epidermidis, Staphylococcus aureus, and Pseudomonas aeruginosa. In this scenario, the patient's symptoms of increasing pain and decreased vision after an initial improvement suggest an infectious process. The presence of an intraocular lens, a foreign body, and the surgical site create an ideal environment for bacterial growth, leading to endophthalmitis.

### **Why Each Wrong Option is Incorrect**
**Option B:** After cataract, also known as posterior capsule opacification, typically presents with gradual decrease in vision and is usually not accompanied by pain.

**Option C:** Central retinal vein occlusion is a retinal vascular event that can cause sudden vision loss, but it is not typically associated with pain and is often accompanied by retinal hemorrhages.

**Option D:** Retinal detachment is a serious condition that can cause sudden vision loss, but it is usually accompanied by symptoms such as flashes of light, floaters, and a sudden increase in eye pressure, not pain.

### **Clinical Pearl / High-Yield Fact**
In cases of suspected endophthalmitis, prompt treatment with intravitreal antibiotics is essential to prevent permanent vision loss. A vitreous tap and culture should be performed to identify the causative organism and guide antibiotic selection.

### **Correct Answer:**
✓ Correct Answer: A. Endophthalmitis