A 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