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
In a patient with AIDS, which of the following organism is the most common cause of chorioretinitis? 
Select and explain. 
A. Cytomegalovirus, B. Toxoplasma gondii, C. Histoplasma capsulatum, D. Cryptococcus neoformans
A: Cytomegalovirus
Answer: A: Cytomegalovirus 
Explanation: CMV retinitis is the most frequent and serious ocular oppounistic infection and is the leading cause of blindness in AIDS patients. In early stages retinitis may be asymptomatic, but later causes changes in visual acuity, visual field cuts, photophobia, scotoma, and eye redness or eye pain. Indirect ophthalmoscopy shows fluffy white perivascular lesions with areas of hemorrhage within them. Treatment includes intraocular ganciclovir implants, in conjunction with oral ganciclovir. Alternative first-line therapy is ganciclovir without implants for 14 to 21 days. Ref: Harrison's Internal Medicine, 18th Edition, Chapter 214 ; Tintinalli's Emergency Medicine: A Comprehensive Study Guide, 7th Edition, Chapter 149