A young male patient has an elevated PTT of 65 seconds and a history of a swollen knee. What is the possible diagnosis based on the blood test results?

The presence of an elevated activated partial thromboplastin time (aPTT) of 65 seconds, particularly in the context of a swollen knee, suggests an issue with the intrinsic pathway of the coagulation cascade. Hemophilia A is specifically characterized by a deficiency in factor VIII, which is critical for proper blood clotting and primarily affects the intrinsic pathway leading to an increased aPTT. The history of a swollen knee may indicate a joint bleed, which is a common manifestation in individuals with hemophilia, as they often experience bleeding into joints and muscles.

It's essential to differentiate this from other conditions. While von Willebrand disease could also lead to a prolonged aPTT, it usually has additional signs such as mucosal bleeding, and it affects both platelet function and factor VIII levels. Fibrinogen deficiency would typically prolong both the aPTT and prothrombin time (PT), which is not indicated in this scenario. Lastly, lupus anticoagulant can cause aPTT elevation but is often associated with other clinical symptoms and backgrounds, such as a history of thrombosis or autoimmune disease, which are not mentioned here. Thus, the clinical presentation and laboratory findings are most consistent with Hemophilia A.