An adult presents with fever after travel to Africa. A peripheral blood smear reveals an extracellular, flagellated parasite. What is the most likely diagnosis?

• A. Trypanosoma brucei
• B. Plasmodium falciparum
• C. Leishmania donovani
• D. Toxoplasma gondii

Answer: (A)

An extracellular, flagellated organism seen in the blood after travel to Africa points to African sleeping sickness caused by Trypanosoma brucei. This parasite circulates in the bloodstream as flagellated trypomastigotes, which are slender, motile, and have a visible flagellum with an undulating membrane. This morphology is distinct from the other organisms: malaria parasites (Plasmodium falciparum) live inside red blood cells as intracellular forms; Leishmania donovani exists as intracellular amastigotes within macrophages; and Toxoplasma gondii is also intracellular, often seen in tissue as tachyzoites or cysts. The blood-stage extracellular flagellated parasite in an Africa-exposure context makes Trypanosoma brucei the most likely diagnosis.