Following a recent hiking trip in Nantucket, Massachusetts, a 48-year-old male is brought … — USMLE Step 3

Following a recent hiking trip in Nantucket, Massachusetts, a $48$ -year-old male is brought to the urgent care clinic by his spouse due to three days of high fevers, drenching sweats, and noticeably dark urine. The patient recently completed a two-week course of oral doxycycline for an expanding, ring-like rash on his right calf. Today, his temperature is $39.4^{\circ}\text{C}$ ( $102.9^{\circ}\text{F}$ ), heart rate is $115/\text{min}$ , and blood pressure is $112/68\text{ mmHg}$ . Physical examination is notable for yellowing of the sclerae and mild left upper quadrant abdominal tenderness with a palpable spleen edge.

Laboratory analysis reveals:
Hemoglobin: $8.8\text{ g/dL}$
Platelets: $92,000/\text{mm}^3$
Leukocytes: $7,200/\text{mm}^3$
Total bilirubin: $3.6\text{ mg/dL}$
Indirect bilirubin: $3.0\text{ mg/dL}$
Lactate dehydrogenase: $780\text{ U/L}$
Prothrombin time (PT): $12\text{ s}$ (normal: $11-13\text{ s}$ )
Activated partial thromboplastin time (aPTT): $26\text{ s}$ (normal: $25-35\text{ s}$ )

Which of the following is the most likely diagnosis?

BabesiosisAnswer
B
Human granulocytic anaplasmosis
C
Treatment-resistant Borrelia burgdorferi infection
D
Autoimmune hemolytic anemia
E
Disseminated intravascular coagulation

Answer

Babesiosis

The correct answer is Babesiosis. This vignette requires formulating a differential diagnosis based on a classic scenario of tick-borne co-infection. The Ixodes scapularis tick transmits Borrelia burgdorferi (Lyme disease), Anaplasma phagocytophilum (Anaplasmosis), and Babesia microti (Babesiosis). The patient's initial presentation of erythema migrans was correctly treated with doxycycline. However, while doxycycline is highly effective against Lyme disease and Anaplasmosis, it has no activity against Babesia. Babesiosis is an intraerythrocytic parasitic infection that causes red blood cell lysis, presenting clinically with high fevers, drenching sweats, jaundice, and dark urine. The laboratory findings of anemia, thrombocytopenia, elevated lactate dehydrogenase, and indirect hyperbilirubinemia confirm an active hemolytic process, which is the hallmark of Babesiosis among tick-borne illnesses.

Step-by-Step Solution

Analyze the patient's epidemiological history and initial presentation.

The patient had a recent tick exposure in a highly endemic area (Nantucket) leading to early localized Lyme disease (erythema migrans).

Establishing the geographic and vector exposure highlights the risk for specific diseases transmitted by the Ixodes scapularis tick.

Evaluate the clinical and laboratory profile of the current symptoms.

The patient has new-onset high fever, jaundice, splenomegaly, and labs demonstrating intravascular hemolysis (low hemoglobin, high LDH, high indirect bilirubin, dark urine) with thrombocytopenia, but normal coagulation studies.

Categorizing the physiological derangement restricts the differential diagnosis strictly to causes of hemolytic anemia and thrombocytopenia.

Synthesize the current presentation with the recent pharmacological intervention.

The patient completed a course of doxycycline. Doxycycline treats Lyme disease and Anaplasmosis but is ineffective against Babesia microti.

Ruling out adequately treated coinfections isolates the correct pathogen that explains both the lab profile and the treatment failure.

Key Concept

Formulating a differential diagnosis for tick-borne coinfections by analyzing therapeutic failure and specific laboratory profiles (hemolysis).

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