## Core Concept
The question tests knowledge of protozoan infections and their specific targets within the human body, particularly focusing on ocular involvement. Protozoa are a group of eukaryotic microorganisms that can cause various diseases in humans. Some protozoa have a specific affinity for certain tissues or organs.

## Why the Correct Answer is Right
Toxoplasma gondii is well-known for causing toxoplasmosis, an infection that can affect various parts of the body, including the eyes. Ocular toxoplasmosis is a significant concern, especially in immunocompromised individuals and congenitally infected newborns. The parasite can cause retinochoroiditis, leading to vision impairment. Toxoplasma gondii's lifecycle involves cats as definitive hosts and a wide range of warm-blooded animals as intermediate hosts. Humans can become infected through exposure to cat feces, contaminated food or water, or congenital transmission.

## Why Each Wrong Option is Incorrect
- **Option A:** While certain species of Leishmania can cause visceral leishmaniasis, which affects internal organs like the spleen, liver, and bone marrow, ocular involvement is not the hallmark or most commonly associated feature of the disease.
- **Option B:** Giardia lamblia primarily causes gastrointestinal infection, leading to giardiasis, characterized by diarrhea, abdominal cramps, and weight loss. Ocular involvement is not typical.
- **Option D:** Plasmodium spp., the causative agents of malaria, primarily affect the liver and red blood cells, leading to anemia, fever, and potentially life-threatening complications. While malaria can have ocular manifestations, such as retinal changes, the primary and most direct association with ocular disease among the given options is not with Plasmodium.

## Clinical Pearl / High-Yield Fact
A key point to remember is that *Toxoplasma gondii* is a major cause of posterior uveitis in immunocompetent individuals and can cause severe ocular disease in immunocompromised patients and congenitally infected infants. A classic clinical scenario involves a pregnant woman with a primary *T. gondii* infection, which can lead to congenital toxoplasmosis in the fetus, manifesting with ocular and systemic abnormalities.

## Correct Answer: C. Toxoplasma gondii