Final answer:
The diagnosis for an eight-year-old boy with hemarthrosis, increased PTT, but normal PT and bleeding time is most likely hemophilia, a genetic clotting disorder. The treatment is usually regular infusions with the specific clotting factor that is deficient.
Step-by-step explanation:
A boy who is eight years old presenting with hemarthrosis (bleeding into joint spaces), prolonged Partial Thromboplastin Time (PTT) with a normal Prothrombin Time (PT) and normal bleeding time is likely suffering from hemophilia, a genetic disorder affecting the clotting process. The most common types are hemophilia A and hemophilia B, caused by deficiencies of clotting factors VIII and IX, respectively. Treatment typically involves regular infusions of the deficient clotting factor, which is derived from healthy donors. Looking to the future, genetic therapy may provide a more definitive treatment option.