Final answer:
Given the family history and lab results indicating a problem with the intrinsic clotting pathway, the next test to perform for the 4-year-old boy's bleeding disorder should be a Factor VIII assay to diagnose the potential for Hemophilia A.
Step-by-step explanation:
Choosing the Next Test for a Bleeding Disorder
A 4-year-old boy with a history of bleeding may have a genetic disorder affecting his clotting factors. Given his family history and lab results, it is essential to diagnose the specific type of hemophilia he may have. The laboratory findings show a prolonged activated partial thromboplastin time (APTT) which suggests a defect in the intrinsic pathway of the coagulation cascade, specifically factors VIII, IX, XI, or XII. Since factor XII deficiency is generally asymptomatic and would not explain the bleeding history, conducting a Factor VIII assay is the next appropriate step. This test will help in diagnosing Hemophilia A, the most common form of hemophilia, which is characterized by a deficiency in Factor VIII and correlates with the provided family history suggestive of an X-linked pattern of inheritance.
The boy's results of a normal platelet function assay (PFA) and prothrombin time (PT) eliminate platelet function abnormalities and extrinsic pathway issues respectively, hence Platelet aggregation tests are not required at this stage. Given that the mixing study did not correct the APTT, a specific factor deficiency is more likely than a circulating anticoagulant, which argues against the immediate need for Circulating anticoagulant studies. Due to the normal PT, Factor XII deficiency is less likely as it is generally not associated with a bleeding diathesis and thus would not typically necessitate immediate testing.