A 26-year-old woman was recently diagnosed with HIV with a CD4 count of 136 cells/mm³. She has a history of developing a rash when taking trimethoprim-sulfamethoxazole, so dapsone has been started for Pneumocystis pneumonia prophylaxis. She returned 1 month later for evaluation of shortness of breath, light-headedness, and difficulty walking more than about 20 feet. On examination, her lips appeared blue. Pulse oximetry reveals an oxygen saturation of 87% on room air, and arterial blood gas demonstrates a PaO2 of 99 mmHg. A blood sample is drawn, and the tube of blood appears chocolate brown in color.