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?