**Core Concept**
Intracellular parasites are microorganisms that live and replicate within host cells, evading the immune system and causing various diseases. These parasites often have complex life cycles, involving multiple stages of development within host cells.

**Why the Correct Answer is Right**
The correct answer is **Toxoplasma gondii**, which is an obligate intracellular parasite that infects a wide range of host cells, including neurons and macrophages. Toxoplasma gondii is unique among the options, as the others are extracellular parasites that infect host cells but do not replicate within them.

**Why Each Wrong Option is Incorrect**
**Option A:** Plasmodium species are intracellular parasites that infect red blood cells and cause malaria. They are obligate intracellular parasites, replicating within the host cell.
**Option B:** Leishmania species are intracellular parasites that infect macrophages and cause leishmaniasis. They are obligate intracellular parasites, replicating within the host cell.
**Option C:** Trypanosoma brucei is an extracellular parasite that infects red blood cells and causes African trypanosomiasis. However, it is not an obligate intracellular parasite, as it can survive and replicate outside host cells.

**Clinical Pearl / High-Yield Fact**
Toxoplasma gondii is a significant zoonotic pathogen, transmitted to humans through contact with contaminated cat feces or undercooked meat. Pregnant women are particularly vulnerable to toxoplasmosis, as the parasite can cause congenital infection and fetal abnormalities.

**Correct Answer:** D. Trypanosoma brucei is an extracellular parasite.