## Core Concept
Sleeping sickness, also known as African trypanosomiasis, is a parasitic disease caused by an infection with protozoan parasites. These parasites are transmitted by the bite of an infected tsetse fly. The disease affects the central nervous system (CNS) in its advanced stage, leading to symptoms such as confusion, personality changes, and difficulty sleeping, hence the name "sleeping sickness."

## Why the Correct Answer is Right
The correct answer, **Trypanosoma brucei**, is the causative agent of African trypanosomiasis or sleeping sickness. This parasite has two subspecies: *Trypanosoma brucei rhodesiense* (found in eastern and southern Africa) and *Trypanosoma brucei gambiense* (found in central and western Africa). The parasite invades the host's bloodstream and eventually crosses the blood-brain barrier to infect the CNS, leading to the neurological symptoms characteristic of the disease.

## Why Each Wrong Option is Incorrect
- **Option A:** *Leishmania donovani* is incorrect because it causes visceral leishmaniasis, a different parasitic disease that primarily affects the liver, spleen, and bone marrow.
- **Option B:** *Plasmodium falciparum* is incorrect because it is one of the parasites responsible for malaria, not sleeping sickness.
- **Option D:** *Toxoplasma gondii* is incorrect because, while it is a protozoan parasite, it causes toxoplasmosis, which has a different clinical presentation and primarily affects immunocompromised individuals and fetuses.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the diagnosis of sleeping sickness involves demonstrating the parasite in the blood, lymph nodes, or CNS. The disease has two stages: the hemolymphatic stage and the neurological stage. The treatment varies depending on the stage and the subspecies of the parasite, emphasizing the need for accurate diagnosis.

## Correct Answer: C. Trypanosoma brucei.