## Core Concept
The class Coccidia includes a group of protozoan parasites that are characterized by their life cycle, which typically involves both asexual and sexual phases. These parasites are usually single-celled and reproduce through the formation of spores. Coccidia are known to infect a wide range of hosts, including humans and animals.

## Why the Correct Answer is Right
Toxoplasma gondii (Option B) is indeed a member of the class Coccidia. It is an obligate intracellular parasite that causes toxoplasmosis. The life cycle of T. gondii involves both asexual and sexual reproduction phases, characteristic of coccidia. On the other hand, *Leishmania* species (Option D) belong to the class Kinetoplastida, not Coccidia. *Leishmania* are flagellated protozoa that cause leishmaniasis, a disease spread by sandflies. The key distinction lies in their morphology, life cycle, and mode of reproduction, which are quite different from those of coccidia.

## Why Each Wrong Option is Incorrect
- **Option A:** *Isospora belli* (also known as *Cystoisospora belli*) is a member of the class Coccidia. It is a protozoan parasite that causes intestinal coccidiosis in humans. Its life cycle and characteristics align with those of the Coccidia class.
- **Option C:** *Cyclospora cayetanensis* is also classified under the class Coccidia. It causes cyclosporiasis, a diarrheal disease, and its life cycle features asexual and sexual phases typical of coccidian parasites.

## Clinical Pearl / High-Yield Fact
A key point to remember is that *Leishmania* species are distinct from coccidia due to their kinetoplast DNA and flagellated forms, and they cause a different spectrum of diseases. This distinction is crucial for diagnosis and treatment planning.

## Correct Answer Line
**Correct Answer: D. Leishmania**.