This is sickle cell anemia in sickle cell crisis. The abnormal hemoglobin SS is prone to form tactoids with crystallization in the RBC's when oxygen tension is low, and the RBC's change shape to long, thin sickle forms that are "sticky" and sludge in capillaries, further decreasing blood flow and oxygen tension. Persons with sickle cell trait (Hemoglobin AS) are much less likely to have this happen.

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