A physician-validated, board-style question from the Active Transport QBank. Try it, then check the reasoning for every option.
A 32-year-old man is brought to the physician by his wife for a 3-day history of fever, headaches, and myalgias. He returned from a camping trip in Oklahoma 10 days ago. He works as a computer salesman. His temperature is 38.1°C (100.6°F). Neurologic examination shows a sustained clonus of the right ankle following sudden passive dorsiflexion. He is disoriented to place and time but recognizes his wife. Laboratory studies show a leukocyte count of 1,700/mm3 and a platelet count of 46,000/mm3. A peripheral blood smear shows monocytes with intracytoplasmic morulae. Which of the following is the most likely causal organism?
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A
Coxiella burnetiiIncorrect. Coxiella burnetii (Q fever) is transmitted by inhalation from livestock/parturient animals, causes pneumonia or hepatitis, and does not produce intracytoplasmic morulae.
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B
Rickettsia rickettsiiIncorrect. Rickettsia rickettsii (RMSF) causes a centripetal petechial rash (wrists/ankles spreading inward), is endothelial-tropic (no intracellular morulae), and would more often show a rash in this presentation.
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C
Anaplasma phagocytophilumIncorrect. Anaplasma phagocytophilum also produces morulae but in NEUTROPHILS (granulocytes), not monocytes; it is also concentrated in the Upper Midwest/Northeast, not Oklahoma.
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D
Ehrlichia chaffeensisCorrect. Ehrlichia chaffeensis infects monocytes, producing intracytoplasmic morulae, and is endemic to Oklahoma and the south-central US via Lone Star tick bites.
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E
Borrelia burgdorferiIncorrect. Borrelia burgdorferi causes Lyme disease (erythema migrans, arthritis, carditis, neuro symptoms) in the Northeast/Upper Midwest, is rare in Oklahoma, and does not produce intracytoplasmic morulae.
↑ Tap an answer to reveal the reasoning
Answer: D. A camper from Oklahoma with fever, headache, leukopenia, thrombocytopenia, and intracytoplasmic morulae in monocytes has human monocytic ehrlichiosis caused by Ehrlichia chaffeensis, transmitted by the Lone Star tick (Amblyomma americanum). The pathognomonic finding is morulae (mulberry-shaped clusters of bacteria within phagosomes) in MONOCYTES — distinguishing it from anaplasmosis, where morulae appear in granulocytes.
Clinical clues: south-central/southeastern US distribution, fever with cytopenias, transaminitis, and CNS involvement (this patient's clonus and disorientation indicate meningoencephalitis, a known severe manifestation). Unlike Rocky Mountain spotted fever, ehrlichiosis typically lacks a prominent rash (rash occurs in <30% of adults).
Treatment is empiric doxycycline started on clinical suspicion, before laboratory confirmation — delayed treatment increases mortality. Doxycycline is given even in children because the risk of severe ehrlichiosis outweighs dental staining concerns.
**Why each option:**
**A.** Coxiella burnetii (Q fever) is transmitted by inhalation from livestock/parturient animals, causes pneumonia or hepatitis, and does not produce intracytoplasmic morulae.
**B.** Rickettsia rickettsii (RMSF) causes a centripetal petechial rash (wrists/ankles spreading inward), is endothelial-tropic (no intracellular morulae), and would more often show a rash in this presentation.
**C.** Anaplasma phagocytophilum also produces morulae but in NEUTROPHILS (granulocytes), not monocytes; it is also concentrated in the Upper Midwest/Northeast, not Oklahoma.
**D.** Correct. Ehrlichia chaffeensis infects monocytes, producing intracytoplasmic morulae, and is endemic to Oklahoma and the south-central US via Lone Star tick bites.