A 3-year-old girl with β-thalassemia major is brought to the emergency department with fever (39.5°C), severe abdominal pain, and splenomegaly. Her hemoglobin is 7.2 g/dL (baseline ~8 g/dL), reticulocyte count is 0.8% (baseline 3–4%), and blood culture is pending. What is the most appropriate immediate next step?