In a patient with AIDS, which of the following organism is the most common cause of chorioretinitis?

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