A 10-month-old male infant presents with acute onset of painful swelling and erythema of both hands and feet over 24 hours. The child was well until 6 months of age. On examination, there is symmetric edema and tenderness of the metacarpals and metatarsals bilaterally. The structure marked **C** in the diagram indicates the typical age of presentation for this condition. Hemoglobin electrophoresis confirms HbSS sickle cell disease. Apart from acute management with hydration and analgesia, which of the following is the most important disease-modifying intervention to prevent recurrent vaso-occlusive crises in this infant?