Which of the following is a differential diagnosis for CMV retinitis?

• A. Histoplasmosis
• B. Toxoplasmosis
• C. Progressive outer retinal necrosis
• D. ARMD

Answer: (C)

The key idea is recognizing conditions that can look the same as CMV retinitis in someone who is severely immunocompromised. Progressive outer retinal necrosis is the best differential because it is another necrotizing retinopathy caused by a herpesvirus (varicella-zoster) that occurs in AIDS and can closely mimic CMV retinitis in appearance and course, especially early on. Both conditions tend to involve the retina with necrosis and can progress rapidly if not treated, which is why they are considered together in the differential.

Histoplasmosis can cause chorioretinal involvement, but it usually presents as granulomatous lesions rather than a necrotizing retinitis pattern typical of CMV. Toxoplasmosis retinitis is a common HIV-related cause of focal necrotizing retinitis, but it generally features prominent vitreous inflammation (vitritis) and a different clinical look. Age-related macular degeneration is not an infectious retinitis at all and presents with macular changes rather than peripheral necrotizing retinal disease.