A 61-year-old male landscape architect presents to the emergency department in coastal Con… — USMLE Step 2 CK

A $61$ -year-old male landscape architect presents to the emergency department in coastal Connecticut during early September. His wife reports that he has had $4$ days of high-grade fever, drenching sweats, profound fatigue, and progressive confusion. His medical history is significant for a splenectomy at age $25$ following blunt abdominal trauma, and essential hypertension. Vital signs show a temperature of $39.8^\circ\text{C}$ ( $103.6^\circ\text{F}$ ), blood pressure of $88/54 \text{ mm Hg}$ , heart rate of $126/\text{min}$ , and respiratory rate of $28/\text{min}$ . Oxygen saturation is $88\%$ on room air. Physical examination reveals marked scleral icterus and pale conjunctivae. Auscultation of the lungs reveals bibasilar crackles.

Laboratory evaluation shows the following:

Test	Value
Hemoglobin	$6.8 \text{ g/dL}$
Platelet count	$72,000/\text{mm}^3$
Leukocyte count	$14,200/\text{mm}^3$
Total bilirubin	$5.2 \text{ mg/dL}$
Direct bilirubin	$0.8 \text{ mg/dL}$
Lactate dehydrogenase	$1,350 \text{ U/L}$
Blood urea nitrogen	$48 \text{ mg/dL}$
Creatinine	$2.6 \text{ mg/dL}$

A peripheral blood smear demonstrates numerous intraerythrocytic pleomorphic ring forms and occasional tetrad structures. Quantitative analysis reveals a parasitemia level of $16\%$ .

Which of the following is the most appropriate management for this patient?

Intravenous clindamycin, oral quinine, and red blood cell exchangeAnswer
B
Oral atovaquone and oral azithromycin
C
Intravenous artesunate and red blood cell exchange
D
Intravenous ceftriaxone and oral doxycycline
E
Plasma exchange and intravenous corticosteroids

Answer

Intravenous clindamycin, oral quinine, and red blood cell exchange

The correct management for this patient is intravenous clindamycin, oral quinine, and red blood cell exchange. This patient's presentation of a tick-borne febrile illness in the Northeastern United States, combined with a blood smear showing intraerythrocytic tetrads, confirms the diagnosis of babesiosis. Because he has a history of splenectomy, parasitemia greater than 10%, pulmonary edema, and renal failure, he meets the criteria for severe babesiosis. Severe disease necessitates immediate red blood cell exchange transfusion to rapidly reduce the parasite load and remove inflammatory cytokines and hemolytic byproducts, administered alongside targeted antimicrobial therapy.

Step-by-Step Solution

Analyze the clinical presentation and epidemiologic risk factors.

A splenectomized patient in the Northeastern United States presents with fever, jaundice, acute kidney injury, and respiratory distress.

Identifying the geographic and host risk factors helps narrow the differential for acute febrile illnesses.

Interpret the laboratory findings and peripheral blood smear.

Labs show severe hemolytic anemia and thrombocytopenia. The blood smear reveals intraerythrocytic tetrads (Maltese cross), which are pathognomonic for Babesia microti.

The blood smear provides the definitive diagnosis, distinguishing it from malaria (which lacks tetrads) and TTP (which has schistocytes).

Assess the severity of the infection.

The patient has severe babesiosis, evidenced by a parasitemia level >10%, asplenia, severe anemia (Hb < 8 g/dL), pulmonary edema, and acute kidney injury.

Determining severity is critical because it dictates whether outpatient antimicrobials are sufficient or if invasive interventional management is required.

Select the appropriate therapeutic regimen based on disease severity.

Severe babesiosis requires immediate red blood cell exchange transfusion combined with intravenous clindamycin and oral quinine (or IV azithromycin and PO atovaquone).

RBC exchange rapidly decreases the parasite burden and removes toxic byproducts of hemolysis, which is life-saving in severe cases.

Key Concept

Severe babesiosis (indicated by parasitemia >10%, Hb <8 g/dL, pulmonary/renal compromise, or asplenia) requires red blood cell exchange transfusion in addition to targeted antimicrobial therapy.

Estimated Time:2m 0s

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