<prompt> 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 AHemophilia BHenoch-Schonlein purpuraIdiopathic thrombocytopenic purpura  <response>: 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 patients blood with that of a normal donor normalizes the PTT.Diagnosis: Hemophilia B 