A Study to Investigate the Course of Synovial Hypertrophy in Patients With Haemophilia A on Efanesoctocog Alfa Prophylaxis

Inclusion Criteria: 1. Capable of giving signed informed consent, which includes compliance with the requirements and restrictions listed in the informed consent form (ICF) and in the protocol. Parents' or legally designated representatives' consent is required for patients who are \<18 years of age or unable to give consent, or as applicable per local laws. Patients who are \<18 years of age should provide assent in addition to the parents'/legally designated representatives' consent, if appropriate. 2. Male or female patients who are ≥12 years of age and diagnosed with moderate or severe haemophilia A (defined as ≤5% of normal FVIII clotting activity) at the time of signing the ICF. 3. A female patient is eligible to participate if she is not pregnant at enrolment and does not plan to become pregnant during the study. A woman of child-bearing potential (WOCBP) must have a negative highly sensitive serum pregnancy test at the Screening Visit. 4. Must have received prophylactic treatment per local label with any marketed FVIII product or emicizumab for ≥12 months prior to the Baseline Visit. 5. Have at least one eligible index joint (ankle, elbow, knee). 6. Have 12 months of documented pre-study treatment data on haemophilia prescriptions and on treated bleeding episodes prior to the Baseline Visit. 7. Willingness and the ability of the patient or their legally designated representative to complete training in the use of the study patient diary and to complete the diary throughout the study. Exclusion Criteria: 1. Blood clotting disorders other than haemophilia A 2. Already on efanesoctocog alfa treatment 3. Positive inhibitor result (assessed by local laboratory) from the Screening Visit, defined as ≥0.6 Bethesda units (BU)/mL. 4. History of inhibitors without successful immune tolerance induction (ITI) * Successful ITI is defined as: * Negative inhibitor titer (\<0.6 BU/mL) * FVIII recovery \> 66% of expected * FVIII half-life ≥ 6 hours 5. ITI performed within the last 2 years prior to the Baseline Visit. 6. Currently receiving treatment with any of the prohibited concomitant medications, as specified by the protocol. 7. Planned major orthopaedic procedure in any eligible index joint during the course of the study. 8. Patients are not eligible for participation in the study if they cannot undergo MRI assessments at the Baseline Visit. 9. Patients with known hypersensitivity to the active substance or to any of the excipients. 10. Patient not suitable for participation, whatever the reason, as judged by the Investigator, including medical or clinical conditions, or patients potentially at risk of noncompliance to study procedures. 11. Enrolment in a concurrent clinical interventional study, or intake of an investigational medicinal product (IMP), within 3 months prior to inclusion in the study.