This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Ophthalmology
A 60 year old diabetic patient has an uneventful phacoemulsification with IOL implantation. He presented with pain, diminution of vision, redness, watering and a grayish yellow pupillary reflex on the third postoperative day. Examination revealed circumcorneal congestion, aqueous cells 4+, hypopyon in the anterior chamber with posterior synechiae and retrolental flare. The most likely diagnosis is: 
Select and explain. 
A. Post-op endophthalmitis, B. Post-op glaucoma, C. Post-op keratitis, D. Post-op cystitis
A: Post-op endophthalmitis
Answer: A: Post-op endophthalmitis 
Explanation: A i.e. Post-op endophthalmitis