This study is checking if a medicine called crovalimab is safe and works well for children with a rare disease called atypical hemolytic uremic syndrome (aHUS). aHUS is a condition that can cause problems in the blood and kidneys. The study will also look at how the body processes crovalimab. To join, kids must weigh at least 5 kg (like a small backpack) and have certain vaccinations. Kids who are already taking other medicines must have been on a stable dose for 28 days. Girls who can have babies must agree to use birth control or not have sex.

• The study requires vaccinations before participating.
• Participants may continue some current medications if they have been stable.
• It's important to have a stable health condition and no recent infections.

Kids who have other kidney diseases, recent infections, or certain conditions can't join. Pregnant girls or those planning to become pregnant are also not allowed. The study aims to help understand if crovalimab can be a good treatment for this rare illness.