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Difficulty: HardFormulating a Differential Diagnosis

A local health department in an endemic region is evaluating several patients who presented with acute febrile illnesses during the summer months. Match each patient's clinical presentation with the most likely diagnosis.

  • A 62-year-old man with a history of hereditary spherocytosis requiring a surgical intervention in childhood presents with a 1-week history of fatigue, fevers, and dark urine. Labs show a hemoglobin of 8.2 g/dL, elevated lactate dehydrogenase, and a normal leukocyte count.Babesiosis
  • A 45-year-old woman presents with fevers, myalgias, and severe headache. Laboratory evaluation reveals a leukocyte count of 2,400/µL, platelet count of 88,000/µL, and elevated aminotransferases. A peripheral blood smear demonstrates morulae within mononuclear cells. She has no rash.Human monocytic ehrlichiosis
  • A 31-year-old man presents with acute onset of severe headache, fevers, and profound myalgias. Laboratory studies show a sodium level of 128 mEq/L and a platelet count of 95,000/µL. On examination, a maculopapular eruption is noted on the palms and soles.Rocky Mountain spotted fever
  • A 54-year-old man presents with progressive fatigue, lightheadedness, and dyspnea on exertion. Examination reveals a heart rate of 42/min and a faint, 8-cm erythematous plaque with central clearing on his posterior thorax. Electrocardiogram confirms complete heart block.Lyme disease

Answer

The presentation of hemolytic anemia in an asplenic patient matches Babesiosis. The presentation of cytopenias with mononuclear morulae matches Human monocytic ehrlichiosis. The presentation of hyponatremia with a palmar rash matches Rocky Mountain spotted fever. The presentation of heart block with an annular rash matches Lyme disease.
Formulating a differential diagnosis for tick-borne diseases requires synthesizing historical risk factors, rash characteristics, and laboratory findings. Babesiosis uniquely causes hemolysis, particularly in asplenic patients. Ehrlichiosis is characterized by cytopenias and morulae in monocytes. Rocky Mountain spotted fever is defined by its centripetal rash involving the palms and soles along with hyponatremia. Lyme disease is distinguished by erythema migrans and potential cardiac involvement (AV block).

Step-by-Step Solution

1
Analyze the first patient's surgical history and current laboratory findings.
Identified prior splenectomy (due to spherocytosis) and active hemolytic anemia (dark urine, high LDH, low hemoglobin).
Splenectomy causes a state of immunodeficiency against intraerythrocytic parasites, making Babesiosis the definitive diagnosis among tick-borne illnesses causing hemolysis.
2
Evaluate the second patient's cytopenias and peripheral smear.
Identified leukopenia, thrombocytopenia, elevated liver enzymes, and morulae in mononuclear cells without a rash.
These findings are highly specific for Human monocytic ehrlichiosis, distinguishing it from Anaplasmosis (which features granulocytic morulae) and diseases that typically cause a rash.
3
Assess the third patient's rash distribution and electrolyte abnormalities.
Identified a rash on the palms and soles combined with hyponatremia and thrombocytopenia.
A rash involving the palms and soles in a tick-endemic area strongly suggests Rocky Mountain spotted fever; hyponatremia is a frequently tested supportive clue due to ADH release or endothelial damage.
4
Interpret the fourth patient's cardiac findings and skin lesion.
Identified complete heart block and erythema migrans (annular lesion with central clearing).
Lyme disease classically presents with erythema migrans and can cause atrioventricular conduction delays (Lyme carditis) during the early disseminated phase.

Key Concept

Differentiating tick-borne illnesses based on classic triad presentations, specific laboratory derangements (e.g., hemolysis vs. cytopenias), and unique physical exam findings.
Estimated Time:2m 0s
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Formulating a Differential Diagnosis

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