A 30-year-old female stores her contact lenses in tap water. She notices deterioration of vision and visits an ophthalmologist, who diagnoses her with severe retinitis. Culture of the water as well as vitreous fluid would most likely reveal which of the following?

**Core Concept**
Acanthamoeba keratitis and retinitis are opportunistic infections caused by Acanthamoeba species, which can be acquired through contact lens use with contaminated water. The pathogen invades the cornea and retina, leading to severe vision loss. This is a classic example of a waterborne pathogen causing a sight-threatening infection in a susceptible host.

**Why the Correct Answer is Right**
The organism enters the eye through the contact lens, where it invades the corneal epithelium and causes keratitis. The amoeba can penetrate the cornea and spread to the vitreous humor, leading to retinitis. Acanthamoeba species are facultative intracellular pathogens that can survive in water sources, including tap water. The presence of amoebic cysts and trophozoites in the culture of the contaminated water and vitreous fluid would confirm the diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Pseudomonas aeruginosa is a common cause of contact lens-related keratitis, but it is not typically associated with retinitis or the presence of amoebic cysts in the vitreous fluid.
**Option B:** Staphylococcus aureus can cause endophthalmitis, but it is not the primary pathogen in this scenario.
**Option C:** Candida albicans can cause fungal endophthalmitis, but it is not the likely pathogen in this case.

**Clinical Pearl / High-Yield Fact**
Acanthamoeba keratitis and retinitis are typically associated with contact lens use in contaminated water sources, and prompt diagnosis and treatment are crucial to prevent severe vision loss.

**Correct Answer:** C. Acanthamoeba

✓ Correct Answer: A. Acanthamoeba