A 3-year-old boy with known HbSS sickle cell disease is brought to the emergency department with sudden pallor, lethargy, and abdominal distension following a mild upper respiratory tract infection. On examination, the structure marked **A** in the diagram is tense, massively enlarged, and reaches the iliac fossa. His blood pressure is 70/40 mmHg, heart rate 170/min, hemoglobin has dropped from baseline 8 g/dL to 3 g/dL, and the reticulocyte count is appropriately elevated. Which of the following best explains the pathophysiology of this acute life-threatening crisis?

Question

Accepted Answer

C. Massive pooling of sickled erythrocytes within splenic cords leading to rapid sequestration of circulating blood volume and hypovolemic shock. ## Why option 1 is correct

In acute splenic sequestration crisis, the structure marked **A** (massively enlarged congested spleen) becomes engorged because sickled red cells become mechanically trapped in the splenic cords and sinusoids. Unlike older children and adults with sickle cell disease whose spleens have undergone autoinfarction, young children (typically 6 months to 5 years) retain functional splenic tissue. The pooling of massive volumes of blood within the spleen causes rapid, life-threatening anemia and hypovolemic shock. The preserved reticulocyte count (not suppressed) is the key diagnostic clue that distinguishes sequestration crisis from aplastic crisis. This pathophysiology is the defining feature of splenic sequestration crisis in young sickle cell patients (Nelson Pediatrics; British Society for Haematology).

## Why each distractor is wrong

- **Option 2 (Acute splenic infarction)**: While splenic infarction occurs in sickle cell disease, it typically happens gradually over years, leading to autoinfarction and a small fibrotic spleen by mid-childhood. Acute infarction does not cause massive splenic enlargement or the acute sequestration of blood volume seen here. This describes chronic splenic changes, not the acute crisis.

- **Option 3 (Splenic rupture with hemolytic crisis)**: Splenic rupture is a rare complication and would present with acute hemorrhagic shock and peritoneal signs. Hemolytic crisis causes elevated indirect bilirubin and high reticulocytes but does not produce the massively enlarged spleen characteristic of sequestration. The clinical picture here is sequestration, not rupture.

- **Option 4 (Extramedullary hematopoiesis)**: While extramedullary hematopoiesis can cause splenomegaly in chronic hemolytic anemias, it develops gradually and does not cause the acute, tense, massively enlarged spleen with sudden hypovolemic shock seen in this 3-year-old. This mechanism explains chronic splenomegaly, not acute sequestration crisis.

**High-Yield:** Splenic sequestration crisis is a disease of young sickle cell children (6 months–5 years) because their spleens have not yet autoinfarcted; the massive acute splenomegaly + Hb drop + preserved reticulocytes + hypovolemic shock = sequestration, not aplastic or hemolytic crisis.

[cite: Nelson Pediatrics; British Society for Haematology]

Answer

A. Splenic rupture secondary to acute hemolytic crisis with uncontrolled intravascular hemolysis

Answer

B. Splenic infiltration by extramedullary hematopoiesis leading to compensatory erythropoiesis and bone pain

Answer

D. Acute splenic infarction due to vaso-occlusive crisis with release of hemoglobin into the circulation

Explanation

Why option 1 is correct

Why each distractor is wrong

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