American Board of Internal Medicine (ABIM) Certification Practice Exam

Which of the following substances indicates a deficiency that could be involved in hemolytic anemia in PNH?

1. CD55 and CD59

2. CD34 and CD19

3. Fibrinogen and factor V

4. Hemoglobin A and HbF

The correct answer is: CD55 and CD59

In the context of paroxysmal nocturnal hemoglobinuria (PNH), the deficiency of CD55 and CD59 is pivotal. These are glycosylphosphatidylinositol (GPI)-anchored proteins that are important for protecting red blood cells from complement-mediated lysis. In PNH, mutations in the PIGA gene lead to the inability to synthesize GPI anchors, which in turn causes a deficiency of CD55 and CD59 on the surface of red blood cells. The absence of CD55 and CD59 results in unregulated complement activity, which can lead to the destruction of erythrocytes, ultimately causing hemolytic anemia. This hemolysis is a hallmark of PNH, which is characterized by episodes of hemoglobinuria (hemoglobin in the urine), especially during the night or in the early morning. The other substances listed do not relate to the pathophysiology of PNH. CD34 and CD19 are markers associated with hematopoietic stem cells and B-lymphocytes, respectively, and are not relevant to hemolysis or PNH. Fibrinogen and factor V pertain to the coagulation pathway, while hemoglobin A and HbF are types