A 55-year-old woman presents with fever that comes and goes, fatigue, and laboratory evidence of anemia. Peripheral blood smear shows small, delicate intracellular ring forms. She recalls backpacking along the Appalachian Trail in the U.S. Which condition is most likely?

• A. Malaria
• B. Babesiosis
• C. Filariasis
• D. Trypanosomiasis

Answer: (B)

The main idea here is recognizing babesiosis, a tick-borne illness that can mimic malaria but has distinctive features in the Northeastern United States. Babesia microti is transmitted by Ixodes ticks found in areas like the Appalachian region. It causes fever that may come and go and anemia from intravascular hemolysis, which fits the patient’s fatigue and lab evidence.

On a peripheral blood smear, babesial parasites are seen as small, delicate ring forms inside red blood cells. A classic clue, when present, is the Maltese cross tetrad appearance, which—though not always seen—strongly points to Babesia rather than other parasites. The geographic clue (Appalachian Trail exposure) further supports babesiosis, since this area is known for its tick-borne Babesia transmission.

Why this fits best over the others: malaria can cause similar febrile illness with intracytoplasmic parasites, but it is typically linked to travel to malaria-endemic regions abroad rather than the Appalachian region, and the hallmark Maltese cross is more characteristic of Babesia than of Plasmodium species. Filariasis involves lymphatic disease from filarial worms and does not present with intracellular ring forms in red blood cells. Trypanosomiasis shows flagellated trypomastigotes in the blood, not the delicate intracytoplasmic rings described here.

So, the findings point most strongly to babesiosis.