Metabolism USMLE Step 1 Practice Question

A 36-year-old Greek male with viral pneumonia presents with acute hemolysis. Vital signs show BP 128/82 mmHg, HR 102/min, RR 22/min, temperature 38.2°C, SpO2 94% on room air. Laboratory studies reveal hemoglobin 9.2 g/dL, reticulocyte count 8.5% (elevated), and elevated indirect bilirubin. Peripheral blood smear shows polychromasia but no Heinz bodies. Over the subsequent week, reticulocytosis increases further. Which precursor in the heme synthesis pathway is essential for newly matured erythrocytes to achieve functional oxygen-carrying capacity?