A 6-year-old boy is brought to the pediatrician for an evaluation of a yellow tint to his skin and eyes that seems to worsen whenever he has a cold. His mother notes that his older brother had his gallbladder removed at age 12. Past medical history is otherwise unremarkable. On physical examination, the child has mild scleral icterus, and the tip of the spleen is palpable 3 cm below the left costal margin.
Laboratory studies reveal the following:
Which of the following is the most likely underlying pathophysiology of this patient's condition?
The correct answer is:
A) Deficiency in the anchor between the red blood cell membrane and the cytoskeleton
This patient is presenting with hereditary spherocytosis, the most common inherited breakdown of the red blood cell membrane. It is most often caused by defects in proteins such as ankyrin, spectrin, or band 3, which serve as the tethers connecting the lipid bilayer to the internal cytoskeleton. Without these anchors, the red blood cell loses small pieces of its membrane over time. This loss of surface area forces the cell to adopt the most mathematically efficient shape, a sphere. These spherocytes are rigid and lack the central pallor of normal biconcave discs. Because they are less flexible, they become trapped and destroyed as they attempt to pass through the narrow fenestrations of the splenic sinusoids, leading to extravascular hemolysis, splenomegaly, and jaundice.
Answer choice B: Inactivation of the sodium-potassium ATPase pump, is incorrect. While electrolyte imbalances can affect cell volume, this is not the primary mechanism of hereditary spherocytosis. The leakiness to sodium seen in hereditary spherocytosis is a secondary effect of the membrane instability, not the root cause.
Answer choice C: Mutation in the gene encoding the catalytic subunit of pyruvate kinase, is incorrect. This describes pyruvate kinase deficiency, which causes a chronic hemolytic anemia due to ATP depletion. However, it typically presents with burr cells (echinocytes) rather than spherocytes and does not characteristically increase the MCHC.
Answer choice D: Oxidative damage to the hemoglobin beta-chain, is incorrect. This describes the mechanism of G6PD deficiency, where oxidative stress leads to Heinz body formation and bite cells. While G6PD causes episodic hemolysis, it does not result in the chronic presence of spherocytes or a high MCHC.
Answer choice E: Substitution of a hydrophobic amino acid in the globin chain, is incorrect. This refers to sickle cell disease, where valine substitutes for glutamic acid) While both are hemolytic anemias, the morphology (sickle cells vs. spherocytes) and the clinical triggers are distinct.
Key Learning Point
Hereditary spherocytosis is characterized by the triad of hemolysis, jaundice, and splenomegaly, often with a family history of early cholecystectomy due to bilirubin stones. The diagnostic hallmark on labs is an elevated MCHC, reflecting mild cellular dehydration and membrane loss. Diagnosis can be confirmed with the eosin-5-maleimide (EMA) binding test or the osmotic fragility test.
