Which agent has a boxed warning for thrombotic events when used as prophylaxis in Hemophilia A?

• A. Desmopressin
• B. Emicizumab
• C. Factor VIII
• D. Platelet transfusion

Answer: (B)

The question tests safety labeling for prophylactic therapies in Hemophilia A, specifically a serious boxed warning for thrombotic events. Emicizumab is a bispecific antibody that substitutes the function of missing factor VIII to prevent bleeds in Hemophilia A, including in patients with inhibitors. A boxed warning exists for emicizumab when it is used concomitantly with activated prothrombin complex concentrates (aPCC) to treat bleeds or during breakthrough bleeding. In that combination, there can be excessive thrombin generation, leading to thrombotic microangiopathy and thrombotic events. This safety signal makes emicizumab the agent with a boxed warning for thrombotic events in this prophylaxis context.

Desmopressin, factor VIII replacement, and platelet transfusion do not carry this specific boxed warning regarding thrombotic events with concurrent bypassing agents. Desmopressin raises endogenous factor VIII and von Willebrand factor and is generally not associated with that boxed risk profile; factor VIII concentrates can cause thrombosis in inappropriate high-dose or underlying conditions but do not have this particular boxed warning tied to prophylaxis with emicizumab; platelet transfusion is not a standard prophylactic approach for this issue and lacks this boxed warning context.