Q. A 35-year-old woman comes to your primary care clinic with complaints of fatigue and easy bruising for the past month. She has a significant medical history of paroxysmal nocturnal hemoglobinuria. Her physical examination reveals petechiae and ecchymosis on her arms and legs. Laboratory results are shown below. The peripheral blood smear shows normocytic red blood cells. Which of the following diagnostic tests would be the most appropriate next step in her management?