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Difficulty: MediumFormulating a Differential Diagnosis

A 38-year-old man visits the outpatient clinic due to a two-week history of profound fatigue, intermittent chills, and dark-colored urine. He works as a forestry technician and recently completed a three-week fieldwork assignment in coastal Massachusetts. His medical history is significant for a splenectomy following a remote motor vehicle collision. His temperature is 38.8∘C38.8^\circ\text{C} (101.8∘F101.8^\circ\text{F}), blood pressure is 110/70 mm Hg110/70\text{ mm Hg}, and heart rate is 108/min108/\text{min}. Physical examination reveals pale conjunctivae and mild scleral icterus. There is no rash, lymphadenopathy, or hepatosplenomegaly.

Laboratory studies show:

TestResult
Hemoglobin8.4 g/dL8.4\text{ g/dL}
Platelets85,000/mm385,000/\text{mm}^3
Leukocytes9,200/mm39,200/\text{mm}^3
Total bilirubin4.1 mg/dL4.1\text{ mg/dL}
Direct bilirubin0.8 mg/dL0.8\text{ mg/dL}
Lactate dehydrogenase720 U/L720\text{ U/L}
Serum creatinine1.1 mg/dL1.1\text{ mg/dL}

Based on the patient's presentation and laboratory findings, which of the following is the most likely diagnosis?

  1. BabesiosisAnswer
  2. B
    Lyme disease
  3. C
    Thrombotic thrombocytopenic purpura
  4. D
    Disseminated intravascular coagulation
  5. E
    Choledocholithiasis

Answer

The most likely diagnosis is babesiosis.
The correct answer is babesiosis. This patient's presentation of fever, fatigue, and jaundice, combined with laboratory evidence of intravascular hemolysis (anemia, indirect hyperbilirubinemia, elevated LDH) and thrombocytopenia, is highly suspicious for babesiosis. The risk is significantly amplified by his recent fieldwork in an endemic area (Massachusetts) and his history of splenectomy. The spleen is critical for clearing infected erythrocytes, so asplenic patients often experience severe, potentially life-threatening babesiosis.

Step-by-Step Solution

1
Analyze the clinical presentation and demographic risk factors.
The patient is a forestry technician with recent exposure to coastal Massachusetts (a highly tick-endemic region) who lacks a spleen.
Identifying geographical and occupational exposures helps narrow down infectious etiologies, while identifying anatomic deficits (asplenia) highlights susceptibility to encapsulated organisms and intraerythrocytic parasites.
2
Interpret the laboratory findings.
The labs reveal anemia, thrombocytopenia, elevated lactate dehydrogenase, and indirect hyperbilirubinemia with normal creatinine.
This specific pattern confirms a hemolytic process with platelet consumption or destruction, steering the differential away from primarily hepatic or biliary obstructive processes.
3
Synthesize data to formulate the final differential diagnosis.
The combination of tick exposure in the Northeast, severe hemolytic anemia, thrombocytopenia, and asplenia heavily favors babesiosis over other tick-borne or hematologic conditions.
Other tick-borne diseases (like Lyme) lack the hemolytic profile, and other hemolytic conditions (like TTP) do not align with the strong epidemiological exposure.

Key Concept

Babesiosis is a tick-borne protozoal illness that infects erythrocytes, causing hemolytic anemia and thrombocytopenia; it should be strongly suspected in patients with relevant geographical exposure (Northeast US) who present with signs of hemolysis, particularly those who are asplenic and therefore at risk for severe disease.
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Formulating a Differential Diagnosis

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