During morning rounds on the medical ward, a consulting physician evaluates a patient admi… — USMLE Step 2 CK

During morning rounds on the medical ward, a consulting physician evaluates a patient admitted overnight for persistent fever, generalized weakness, and dark urine. The patient was seen at an urgent care center one week ago for an erythematous, expanding annular rash with central clearing on the right thigh following a camping trip in Massachusetts. Oral doxycycline was initiated at that time. Although the rash has faded, the patient reports worsening systemic symptoms. Current vital signs include a temperature of $39.2^\circ\text{C}$ ( $102.6^\circ\text{F}$ ), blood pressure of $110/70 \text{ mmHg}$ , and a heart rate of $112/\text{min}$ . Physical examination is notable for scleral icterus and mild splenomegaly. Laboratory findings show a hemoglobin level of $7.8 \text{ g/dL}$ , platelet count of $95,000/\text{mm}^3$ , elevated indirect bilirubin, and significantly elevated lactate dehydrogenase. A peripheral blood smear reveals pleomorphic intraerythrocytic organisms, with occasional tetrad formations. Which of the following is the most appropriate modification to this patient's antimicrobial regimen?

Add atovaquone and azithromycinAnswer
B
Switch to intravenous ceftriaxone
C
Add intravenous artesunate
D
Switch to oral amoxicillin
E
Add vancomycin and cefepime

Answer

The most appropriate modification is to add atovaquone and azithromycin to the patient's current regimen.

The clinical scenario describes a patient initially treated for early localized Lyme disease who subsequently develops signs of hemolytic anemia (jaundice, dark urine, anemia, elevated LDH and indirect bilirubin) and thrombocytopenia. The Ixodes scapularis tick, which transmits Borrelia burgdorferi, also serves as the vector for Babesia microti and Anaplasma phagocytophilum, making coinfections common in endemic areas like the Northeastern United States. The peripheral blood smear definitively diagnoses babesiosis by demonstrating pleomorphic intraerythrocytic organisms that occasionally form tetrads (the pathognomonic 'Maltese cross' appearance). The most appropriate management is to continue the current course of doxycycline to treat the Lyme disease and concurrently add atovaquone and azithromycin, which is the first-line therapy for mild-to-moderate babesiosis.

Step-by-Step Solution

Analyze the patient's initial presentation and geographic risk.

The patient had an expanding annular rash (erythema migrans) after camping in Massachusetts, a highly endemic area for Ixodes scapularis ticks, establishing a diagnosis of early localized Lyme disease (Borrelia burgdorferi).

Establishing the initial diagnosis explains the rationale for the patient's current doxycycline prescription.

Evaluate the new-onset symptoms and laboratory findings.

The patient developed dark urine, jaundice, severe anemia (

7.8 \text{ g/dL}

), elevated indirect bilirubin, and elevated lactate dehydrogenase, indicating intravascular hemolysis.

Identifying the pathophysiologic mechanism (hemolysis) narrows the differential diagnosis of tick-borne illnesses.

Interpret the peripheral blood smear in the context of the clinical picture.

The smear shows intraerythrocytic organisms with tetrad formations (Maltese crosses). Combined with the hemolytic anemia and the Ixodes tick exposure, this confirms a coinfection with Babesia microti.

Definitive pathogen identification is required to select the targeted antimicrobial therapy.

Select the optimal pharmacotherapy.

The standard treatment for mild-to-moderate babesiosis is a combination of atovaquone and azithromycin. Doxycycline should be continued to complete the course for Lyme disease.

Targeted antimicrobial therapy must cover both established pathogens without unnecessarily escalating to broad-spectrum agents.

Key Concept

Patients residing in or traveling to endemic regions (e.g., Northeastern United States) can acquire coinfections transmitted by the Ixodes scapularis tick. The presence of hemolytic anemia and intraerythrocytic tetrads in a patient being treated for Lyme disease indicates a Babesia microti coinfection, requiring the addition of atovaquone and azithromycin.

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