American Board of Internal Medicine (ABIM) Certification Practice Exam

Fever, leukopenia, and thrombocytopenia in a Lyme disease patient suggest coinfection with which organism?

1. Anaplasma phagocytophilum

2. Borrelia burgdorferi

3. Rickettsia rickettsii

4. Babesia microti

The correct answer is: Anaplasma phagocytophilum

The combination of fever, leukopenia, and thrombocytopenia in a patient with Lyme disease raises the suspicion of coinfection with Anaplasma phagocytophilum. This organism is a tick-borne bacterium that can cause human granulocytic anaplasmosis (HGA), a condition associated with similar hematological findings. Fever is a common systemic manifestation of this infection, while leukopenia and thrombocytopenia are notable laboratory findings that help differentiate it from other infections. Anaplasma phagocytophilum is often transmitted by the same ticks that spread Lyme disease, particularly the Ixodes scapularis tick in the northeastern United States. This overlap in vectors explains the possibility of concurrent infections. While Borrelia burgdorferi is the causative agent of Lyme disease itself, it does not typically present with leukopenia or thrombocytopenia as primary features. Rickettsia rickettsii causes Rocky Mountain spotted fever, which has distinct clinical manifestations, including a rash, but does not typically present with the same pattern of hematological abnormalities as seen in Anaplasma infection. Babesia microti, a protozoan carried by ticks, can also cause thrombocytopen