A 12-year-old boy is brought to the emergency department for evaluation of sudden, painful swelling of his right elbow after a minor fall during physical education class. He has a history of prolonged bleeding following a circumcision in infancy and recurrent deep bruises in his muscles. His maternal grandfather and a male cousin have a similar history of joint swelling and receive regular intravenous infusions for a "bleeding disorder." Vital signs are within normal limits. On physical examination, the right elbow is swollen, warm, and held in a flexed position to minimize pain.
Laboratory studies reveal the following:
Platelet count: 215,000/μL
Prothrombin time (PT): 12 seconds (Normal: 11–13.5s)
Partial thromboplastin time (PTT): 65 seconds (Normal: 25–35s)
Bleeding time: Normal
Factor VIII activity: 100% (Normal)
Which of the following is the most likely primary defect in this patient?
The correct answer is:
B) Deficiency of a clotting factor in the intrinsic pathway
This patient is presenting with Hemophilia B. It is an X-linked recessive disorder caused by a deficiency of Factor IX. Along with Factor VIII, Factor IX is a vital component of the intrinsic pathway of the coagulation cascade, specifically forming part of the tenase complex that activates Factor X. Clinically, Hemophilia B is indistinguishable from Hemophilia A (Factor VIII deficiency). Both present with secondary hemostasis defects: deep tissue bleeding, such as hemarthrosis (joint bleeding) and intramuscular hematomas, rather than the mucosal bleeding seen in platelet disorders. Because the intrinsic pathway is affected, the partial thromboplastin time (PTT) is prolonged, while the PT and platelet count remain normal.
Answer choice A: Deficiency of a clotting factor in the common pathway, is incorrect. The common pathway includes Factors X, V, II (prothrombin), and I (fibrinogen). A deficiency in any of these would typically prolong both the PT and the PTT.
Answer choice C: Impaired formation of the platelet plug, is incorrect. This refers to primary hemostasis disorders like von Willebrand disease or immune thrombocytopenic purpura (ITP). These present with petechiae and mucosal bleeding (epistaxis, gingival bleeding) and an abnormal bleeding time, rather than the spontaneous joint swelling seen in this patient.
Answer choice D: Inability to activate the extrinsic pathway, is incorrect. The extrinsic pathway involves Factor VII and is measured by the PT. A Factor VII deficiency would prolong the PT while the PTT remains normal.
Answer choice E: Qualitative defect in the stabilization of the Factor VIII complex, is incorrect. This describes von Willebrand disease (vWD). While vWD can lead to low Factor VIII levels and a prolonged PTT, it is characterized by an autosomal dominant inheritance and mucocutaneous bleeding. Hemophilia B specifically involves a deficiency of Factor IX.
Key Learning Point
Hemophilia B is a deficiency of Factor IX. It is an X-linked recessive disorder that presents with a prolonged PTT and normal PT and platelet count. The clinical hallmark is hemarthrosis. It can only be distinguished from Hemophilia A by specific factor activity assays.
