An 8-year-old child from Central America presents with pallor and dyspnea; a peripheral blood smear shows an extracellular parasite with an anterior flagellum and a prominent nucleus. Which organism is most likely?

• A. Leishmania braziliensis
• B. Trypanosoma cruzi
• C. Dracunculus medinensis
• D. Trypanosoma brucei

Answer: (B)

This question tests recognizing a bloodstream flagellate consistent with a Trypanosoma species and tying it to the regional parasite likely to be encountered. An extracellular parasite seen on a peripheral smear with an anterior flagellum and a prominent nucleus matches the bloodstream trypomastigote form of Trypanosoma cruzi. In Central America, T. cruzi causes Chagas disease, and pallor with dyspnea can reflect anemia or cardiac involvement seen in Chagas. The parasite is transmitted by triatomine bugs (kissing bugs), which bite and defecate, allowing the parasite to enter through the bite wound. By contrast, Trypanosoma brucei causes African sleeping sickness in sub-Saharan Africa; Leishmania braziliensis tends to present with intracellular amastigotes in macrophages rather than extracellular blood forms; and Dracunculus medinensis is a skin-dwelling nematode, not a blood parasite.