Patient description: The patient is a 23-year-old man who came to the hospital with high blood pressure (175/95 mmHg) and signs of septicemia. He developed respiratory failure requiring mechanical ventilation and renal failure requiring hemodialysis. His blood smear showed microangiopathic hemolytic anemia and thrombocytopenia. His blood tests revealed elevated lactate dehydrogenase and reduced human complement C3 levels with a normal coagulation profile. He was diagnosed with atypical hemolytic uremic syndrome. He was treated with plasma exchange and corticosteroids. He has been previously vaccinated with meningococcal group ACWY conjugate vaccine and meningococcal group B vaccine. The genetic survey revealed ADAMTS13 >10%.

Clinical trial eligibility criteria: eligible ages (years): 0.0-17.0,  Atypical Hemolytic Uremic Syndrome (aHUS) Complement Inhibitor Treatment Naïve: 1. Participants from birth up to <18 years of age and weighing ≥5 kilograms (kg) at the time of consent. 2. Participants had not been previously treated with complement inhibitors. 3. Evidence of thrombotic microangiopathy (TMA), including low platelet count, hemolysis (breaking of red blood cells inside of blood vessels), and decreased kidney function. 4. Documented meningococcal vaccination not more than 3 years prior to dosing, and vaccination against Streptococcus pneumoniae and Haemophilus influenzae type b. 5. Female participants of childbearing potential and male participants with female partners of childbearing potential must have used highly effective contraception starting at screening and continuing until at least 8 months after the last dose of ravulizumab. Eculizumab Experienced: 1. Participants between 12 and <18 years of age (non-Japanese sites) or <18 years of age (Japanese sites) who had been treated with eculizumab according to the labelled dosing recommendation for aHUS for at least 90 days prior to screening. 2. Participants with documented diagnosis of aHUS. 3. Participants with clinical evidence of response to eculizumab indicated by stable TMA parameters at screening. 4. Documented meningococcal vaccination not more than 3 years prior to dosing, and vaccination against Streptococcus pneumoniae and Haemophilus influenzae type b. 5. Females of childbearing potential and male participants with female partners of childbearing potential must have used highly effective contraception starting at screening and continuing until at least 8 months after the last dose of ravulizumab  Known familial or acquired ("a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13") deficiency (activity <5%). 2. Known Shiga toxin-related hemolytic uremic syndrome. 3. Positive direct Coombs test. 4. Females who planned to become pregnant during the study or were currently pregnancy or breastfeeding. 5. Identified drug exposure-related hemolytic uremic syndrome. 6. Bone marrow transplant/hematopoietic stem cell transplant within the last 6 months prior to the start of screening. 7. Hemolytic uremic syndrome related to known genetic defects of cobalamin C metabolism. 8. Known systemic sclerosis (scleroderma), systemic lupus erythematosus, or antiphospholipid antibody positivity or syndrome. 9. Chronic dialysis (defined as dialysis on a regular basis as renal replacement therapy for end-stage kidney disease. 10. For eculizumab-experienced participants, prior use of complement inhibitors other than eculizumab. 11. For eculizumab-experienced participants, any known abnormal TMA parameters within 90 days prior to screening