A 17 year old woman notices that her urine becomes red after she is given sulfonamides for treatment of a urinary tract infection. Both urine and serum test positive for free hemoglobin, and the urine red cell count is 1.2 million/mm3. A peripheral blood smear showed normocytic and normochromic red cells and a few "bite cells." Deficiency of which of the following substances is most likely responsible for these symptoms?

The presence of free hemoglobin in the serum and urine, and "bite cells" due to splenic removal of Heinz bodies (oxidized hemoglobin) all point to hemolysis. Hemolysis following oxidant injury by drugs (sulfonamides, for example) or infection suggests glucose-6-phosphate dehydrogenase deficiency or the related deficiencies of glutathione synthase, pyruvate kinase, and hexokinase. These conditions are typically asymptomatic between episodes of hemolysis. Deficiencies of the alpha and beta chains of hemoglobin produce alpha and beta thalassemia, respectively. Deficiencies of glycoprotein IIb/IIIa produce thrombasthenia, a platelet aggregation defect. Ref: Murray R.K. (2011). Chapter 52. Red & White Blood Cells. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds), Harper's Illustrated Biochemistry, 29e.