A 3-year-old boy presents with a 6-month history of spontaneous hemarthrosis, intramuscular hematomas, and epistaxis. His mother reports no significant bleeding history. Laboratory studies show prolonged activated partial thromboplastin time (aPTT) at 68 seconds (normal 25-35), normal prothrombin time, and normal platelet count. Factor assays reveal factor IX activity of 5% (normal 75-125%). His 9-month-old asymptomatic sister is found to have factor IX activity of 35% on routine preoperative screening before a scheduled hernia repair. Genetic testing confirms a hemizygous factor IX mutation in the boy and a heterozygous mutation for the same factor IX variant in the girl. Which of the following best explains why the sister demonstrates clinical and laboratory features intermediate between an unaffected individual and her affected brother?

1. A)The sister inherited a second independent de novo mutation on her normal X chromosome, making her functionally homozygous
2. B)Maternal germline mosaicism resulted in the sister inheriting a different factor IX mutation on her other X chromosome
3. C)X-linked dominant inheritance pattern of factor IX deficiency, which is partially penetrant in heterozygous females
4. D)Nonrandom X-inactivation (lyonization) with preferential silencing of the X chromosome carrying the normal factor IX alleleGABARITO
5. E)Somatic mosaicism in the mother resulting in variable expression of mutant factor IX in her daughter