A 22-year-old man comes to the emergency depament because of dyspnea, palpitations, and a headache. These symptoms came on soon after he took trimethoprim-sulfamethoxazole for a urinary tract infection. Laboratory studies show a normochromic, normocytic anemia. A peripheral blood smear reveals Heinz bodies. Which of the following is the most likely cause of this patient's anemia?

This patient has glucose-6-phosphate dehydrogenase (G6PD) deficiency, which is an X-linked disorder that leads to hemolytic crises within hours of exposure to oxidant stress. The most common stressors are viral and bacterial infections, sulfa drugs, quinines, and a beans. During an acute hemolytic crisis, hemoglobin becomes denatured and leads to the formation of Heinz bodies. The diagnosis is made by the demonstration of Heinz bodies during an acute crisis, and low levels of G6PD during normal times. The treatment includes maintaining adequate urine output and the prevention of future episodes. Lead poisoning (choice A) leads to a normochromic, normocytic anemia with basophilic stippling. The clinical features include abdominal pain, headache, irritability, and peripheral motor neuropathy. Treatment includes the use of chelating agents. Folate deficiency (choice B) leads to megaloblastic anemia and is most common in alcoholics. Hereditary spherocytosis (choice D) is an inherited membrane disorder that leads to hemolytic anemia and red blood cell swelling. Small, round, hyperchromatic red cells without a central area of pallor are seen on blood smears.