Splenic Sequestration Crisis - Sickle Cell in Children

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?