Okay, let's tackle this question about the parasitic cause of uveitis. First, I need to recall what uveitis is. Uveitis is inflammation of the uvea, which includes the iris, ciliary body, and choroid. It can be caused by various factors, including infections, autoimmune diseases, and trauma.

The question is asking specifically about a parasitic cause. Common parasites that can lead to ocular issues include Toxoplasma gondii, which is known for causing toxoplasmosis. Toxoplasma can lead to ocular manifestations such as uveitis, especially in immunocompromised individuals or congenitally in newborns. Another possibility might be Onchocerca volvulus, which causes river blindness and can affect the eyes, but I'm less sure if it's a common cause of uveitis. Echinococcus granulosus, causing hydatid disease, might also affect the eye, but that's more of a cystic lesion. Schistosoma species can cause uveitis as well, but I need to verify if they are considered a primary cause compared to Toxoplasma.

Wait, the options aren't given here, but the correct answer is supposed to be Toxoplasma gondii. Let me confirm: Toxoplasma is a protozoan parasite, not a helminth. It's a common cause of posterior uveitis, especially in immunocompromised patients. The infection can lead to necrotizing retinitis and uveitis. Other parasites like Toxocara canis (causing ocular larva migrans) might present with uveitis too, but Toxoplasma is more classic. So, the core concept here is identifying the parasitic organisms that can lead to uveitis, with Toxoplasma being the most likely answer. The other options, if they include organisms like Toxocara, Schistosoma, or Onchocerca, would be incorrect because they either cause different types of ocular issues or are not primary causes of uveitis. The clinical pearl would be to remember Toxoplasma as a key parasitic cause of uveitis, especially in the context of immunosuppression.

**Core Concept**
Parasitic uveitis is commonly caused by *Toxoplasma gondii*, a protozoan parasite that infects the retina and choroid, leading to necrotizing retinochoroiditis. Other parasites like *Toxocara canis* or *Schistosoma* species may rarely cause uveitis, but *Toxoplasma* is the most frequent and clinically significant etiology.

**Why the Correct Answer is Right**
*Toxoplasma gondii* causes uveitis through immune-mediated inflammation triggered by bradyzoites in ocular tissue. In immunocompetent individuals, it presents as focal retinal necrosis with white satellite lesions. In immunocompromised patients (e.g., HIV), it progresses rapidly. Diagnosis relies on PCR, serology, or ophthalmoscopic findings like "headlight in a fog" appearance. Treatment includes pyrimethamine, sulfadiazine, and leucovorin.

**Why Each Wrong Option is Incorrect**

✓ Correct Answer: A. Toxocara