A 4-year-old boy develops severe bleeding into the knee joint. Laboratory studies show that serum levels of factor IX are reduced, but levels of factor VIII are normal. What is the appropriate diagnosis?

Hemophilia B is an X-linked recessive disease caused by mutations in the gene encoding factor IX. It accounts for only 10% of all cases of hemophilia. One third of all cases represent new mutations. It is clinically indistinguishable from hemophilia A (factor VIII deficiency) (choice A). In both forms of hemophilia, the partial thromboplastin time (PTT) is prolonged. Mixing of a patient's blood with that of a normal donor normalizes the PTT.Diagnosis: Hemophilia B