A 6-year-old boy with known hereditary spherocytosis, previously stable with baseline Hb 10.5 g/dL, presents after a febrile upper respiratory illness with sudden pallor and lethargy. Hb is 4.2 g/dL, reticulocyte count is 0.2%. The most likely cause of this deterioration is:

• A Hyperhemolytic crisis due to complement activation
• B Transient aplastic crisis caused by parvovirus B19 ✓
• C Splenic sequestration crisis
• D Folate deficiency superimposed on chronic hemolysis

Explanation

Parvovirus B19 infects erythroid progenitor cells via the P antigen and shuts down red cell production for roughly a week. In chronic hemolysis, where red cell survival is short, this causes abrupt, life-threatening anemia with a profoundly low reticulocyte count. A hyperhemolytic crisis would show very high reticulocytes, and splenic sequestration is seen in infants with massive splenomegaly rather than a suppressed marrow picture.

Reference: Nelson Textbook of Pediatrics, 22nd ed.

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Written and medically reviewed by the StethoPrep medical team.