A 12-year-old boy presents to the pediatric clinic with fatigue, recurrent nosebleeds, and easy bruising over the past month. His parents report he has been less active recently and noticed pale skin. There is no history of recent illness, medication use, or significant environmental exposures. His family history reveals a maternal uncle who died of bone marrow failure in his teens. On examination, the boy appears pale and has petechiae on his lower limbs but no lymphadenopathy or hepatosplenomegaly. Laboratory studies reveal pancytopenia, with low hemoglobin, platelets, and white blood cell counts. Bone marrow biopsy shows hypocellular marrow with fat replacement, consistent with aplastic anemia. Genetic testing identifies a mutation, confirming the diagnosis of a hereditary form of aplastic anemia. What is the most common hereditary cause of aplastic anemia?
