**Core Concept**
Late-onset endophthalmitis after lens implantation is a rare but serious complication, often occurring weeks to months after cataract surgery. It is a type of intraocular infection that involves the interior of the eye, including the lens implant.

**Why the Correct Answer is Right**
The most common cause of late-onset endophthalmitis is _Staphylococcus epidermidis_, a coagulase-negative staphylococcus that is naturally present on the skin and can contaminate the intraocular lens implant during surgery. This bacteria can adhere to the lens implant and cause a biofilm to form, leading to a chronic infection. The pathogenesis involves the bacteria's ability to evade the host immune system by producing extracellular polysaccharide matrices and biofilm-related proteins.

**Why Each Wrong Option is Incorrect**
* **Option A:** _Staphylococcus aureus_ is a common cause of endophthalmitis, but it is more commonly associated with acute postoperative endophthalmitis rather than late-onset endophthalmitis.
* **Option B:** _Pseudomonas aeruginosa_ is a rare cause of endophthalmitis and is more commonly associated with contact lens-related infections.
* **Option D:** _Fusarium_ is a type of fungus that can cause endophthalmitis, but it is more commonly associated with traumatic injuries or contact lens-related infections.

**Clinical Pearl / High-Yield Fact**
To minimize the risk of late-onset endophthalmitis, it is essential to maintain strict asepsis during cataract surgery and to use antibiotic-coated intraocular lens implants. Additionally, patients should be instructed to use topical antibiotics after surgery to reduce the risk of bacterial contamination.

**Correct Answer:** C. _Staphylococcus epidermidis_