A 28-year-old man of North Indian descent presents to the emergency department with acute hemolytic anemia (hemoglobin 7.2 g/dL, reticulocyte count 12%), jaundice, and dark urine after taking sulfonamide antibiotics for a urinary tract infection. Peripheral blood smear shows Heinz bodies. He reports a family history of similar episodes triggered by infections and certain drugs. Which investigation is most specific for confirming the underlying glycolytic enzyme deficiency?