Which statement correctly matches the disorder with its key feature?

• A. Hemophilia A and B involve deficiencies of coagulation factors VIII and IX
• B. Von Willebrand disease involves excess platelet aggregation
• C. Thrombocytopenia is defined by elevated platelet counts
• D. Sickle cell disease is primarily a vascular disorder without hematologic effects

Answer: (A)

Disorders of bleeding are best understood by where the problem lies: in the coagulation cascade or in platelets. Hemophilia A and hemophilia B are classic examples of coagulation-factor deficiencies. In one, the body lacks factor VIII; in the other, factor IX. Without these factors, the intrinsic pathway of coagulation cannot properly generate a stable fibrin clot, leading to bleeding tendencies that are often deep and extensive. Lab testing typically shows a prolonged aPTT with a normal PT and a normal platelet count, reflecting a clotting-sauce defect rather than a platelet or vascular problem.

Von Willebrand disease, by contrast, stems from a defect in von Willebrand factor, which is crucial for platelets to adhere to the damaged vessel wall. This impairs platelet plug formation and leads to mucocutaneous bleeding, but it is not characterized by excess platelet aggregation. Thrombocytopenia is the opposite scenario: too few platelets, not elevated counts. Sickle cell disease is a hematologic disorder involving abnormal hemoglobin that causes hemolysis and vaso-occlusion, so its primary impact is not a simple vascular defect devoid of hematologic effects.

So the statement linking hemophilia A and B to deficiencies of coagulation factors VIII and IX correctly captures the essential mechanism.