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Difficulty: HardPediatric Hematology and Oncology

A 6-year-old boy is brought to the emergency department by his parents due to progressive lethargy, weakness, and severe pallor over the past 2 days. One week ago, he had a low-grade fever, rhinorrhea, and a brightly erythematous rash on both cheeks that spared his nasal bridge. His past medical history is significant for neonatal jaundice requiring 4 days of phototherapy. His father underwent a cholecystectomy at age 22. Vital signs show a temperature of 37.1°C (98.8°F), heart rate of 135/min, and blood pressure of 90/60 mm Hg. Physical examination reveals marked conjunctival pallor and a palpable spleen 4 cm below the left costal margin. Laboratory studies reveal the following:

Laboratory testResult
Hemoglobin4.5 g/dL
Mean corpuscular volume (MCV)81 fL
Mean corpuscular hemoglobin concentration (MCHC)37.5 g/dL
Platelets250,000/mm³
Leukocytes7,200/mm³
Reticulocytes0.1%

Which of the following is the most likely primary mechanism for this patient's current severe anemia?

  1. Viral-induced arrest of erythroid progenitor cellsAnswer
  2. B
    Accelerated macrophage-mediated destruction of erythrocytes
  3. C
    Depletion of required micronutrients for DNA synthesis
  4. D
    Rapid pooling of circulating erythrocytes within the splenic parenchyma
  5. E
    Autoantibody-mediated complement activation and intravascular hemolysis

Answer

Viral-induced arrest of erythroid progenitor cells
The correct answer describes the pathogenesis of an aplastic crisis caused by Parvovirus B19. The patient's underlying hereditary spherocytosis is indicated by the family history of early gallstones, neonatal jaundice requiring phototherapy, splenomegaly, and an elevated mean corpuscular hemoglobin concentration (MCHC). The recent 'slapped cheek' rash strongly points to a recent Parvovirus B19 infection. This virus has a strong tropism for erythroid precursors in the bone marrow, where it replicates and causes cell death, leading to a temporary halt in erythropoiesis. In a healthy individual, this transient arrest is usually asymptomatic. However, in a patient with a chronic hemolytic anemia like hereditary spherocytosis—who relies on continuous, high-volume red blood cell production to maintain baseline hemoglobin levels—this sudden halt results in a rapid, severe anemia characterized by an inappropriately low reticulocyte count.

Step-by-Step Solution

1
Identify the underlying chronic condition from the patient's history and baseline laboratory clues.
The history of neonatal jaundice, early-onset gallstones in the father (pigment stones), splenomegaly, and an elevated MCHC (>36 g/dL) strongly point to Hereditary Spherocytosis.
Recognizing the chronic hemolytic baseline is essential to understanding why the patient decompensated so rapidly.
2
Identify the acute precipitating event from the recent clinical history.
The recent febrile illness with a brightly erythematous rash on the cheeks ('slapped cheek' appearance) is the classic presentation of Erythema infectiosum, caused by Parvovirus B19.
The trigger explains the acute change in the patient's clinical status.
3
Correlate the infectious trigger with the current laboratory findings to determine the mechanism of anemia.
Parvovirus B19 infects erythroid precursors in the bone marrow, temporarily halting red blood cell production. In a patient with chronic hemolysis (like Hereditary Spherocytosis), this leads to a rapid, severe drop in hemoglobin accompanied by an inappropriately low reticulocyte count (0.1%).
This confirms the diagnosis of an aplastic crisis rather than a hemolytic or sequestration crisis.

Key Concept

Aplastic crisis in Hereditary Spherocytosis secondary to Parvovirus B19 infection.
Estimated Time:1m 30s
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Topics

Pediatric Hematology and Oncology

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