Emicizumab vs. Efanesoctocog AlfaA Prospective Assessment of Bone Health in Patients With Severe Hemophilia A on Factor VIII vs Factor Mimetic Prophylaxis (Efa Emi Bone Health Study)

Inclusion Criteria:

1. The participant or legally authorized representative is willing and able to provide written informed consent.
2. Diagnosis of severe hemophilia A (factor VIII activity < 1%).
3. Male sex.
4. Age between 30 and 50 years (inclusive).
5. BMI between 18.5 and 40 kg/m2
6. The participant must have been on prophylaxis with Efanesoctocog alfa or Emicizumab for at least 3 months prior to enrollment and intend to remain on the current regimen for the next 5 years.
7. Willingness to undergo all research procedures, including DEXA scans and the collection of blood samples.
8. Willingness to complete all standard-of-care bleeding and treatment logs.

Exclusion Criteria:

1. Unwillingness of the participant, parent, or legally authorized representative to provide informed consent.
2. Diagnosis of a bleeding disorder other than or in addition to severe hemophilia A.
3. Active Factor VIII inhibitors at the time of enrollment
4. History of a disease known to influence bone metabolism unrelated to a bleeding disorder. (Examples: Paget's disease, osteogenesis imperfecta, Ehlers Danlos syndrome, Hyperparathyroidism)
5. Past or present treatment with any anti-osteoporotic medication, excluding oral vitamin D or oral calcium supplements.
6. Documented HIV infection or HCV infection (whether in progress or cured) at the cirrhotic stage.
7. Presence of a non-removable metal device that would interfere with research procedures.
8. Inability to tolerate a DEXA scan due to limited range of motion or body habitus.
9. History of bone fractures or surgical repair within 8 weeks prior to enrollment.
10. Participants with weight >300 pounds, due to limitations of DEXA scanner