Glucose-6-phosphate dehydrogenase (G6PD) is the rate-limiting enzyme of the hexose monophosphate shunt (pentose phosphate pathway), which is the ONLY source of NADPH in red blood cells. NADPH is essential for regenerating reduced glutathione (GSH) via glutathione reductase — GSH is the primary antioxidant defense in erythrocytes, neutralizing hydrogen peroxide (H2O2) and other reactive oxygen species (ROS) that would otherwise denature hemoglobin and damage the RBC membrane. G6PD deficiency is an X-linked recessive enzymopathy (gene on Xq28) affecting approximately 400 million people worldwide, with highest prevalence in malaria-endemic regions (Africa, Mediterranean, Middle East, Southeast Asia) due to heterozygote advantage against Plasmodium falciparum. When G6PD-deficient RBCs encounter oxidative stress (from drugs like primaquine, sulfonamides, rasburicase; infections; fava beans; or DKA), inadequate NADPH production leads to GSH depletion, causing oxidative denaturation of hemoglobin into Heinz bodies (insoluble hemoglobin precipitates that attach to the inner RBC membrane). Splenic macrophages phagocytose the Heinz body-containing portions of the membrane, producing characteristic 'bite cells' on peripheral smear. Severe oxidative damage leads to intravascular hemolysis with hemoglobinemia, hemoglobinuria (dark/cola-colored urine), and unconjugated hyperbilirubinemia. The WHO classifies G6PD variants...