**Study Overview:** This study tests a medicine called *ravulizumab* for people with *chronic kidney disease* (CKD) having heart surgery. The goal is to see if one dose of ravulizumab, given through a vein (IV), can lower the risk of having kidney problems after surgery. This is compared to a placebo, which is a harmless substance that looks like the medicine but has no real effect. The doctors will check the kidneys for 90 days after the surgery. **Key Points:** - Participation requires one IV dose of ravulizumab or placebo before surgery. - Participants with CKD having certain heart surgeries are eligible, but emergency surgeries are not included. - Being in the study may require vaccination and taking antibiotics to prevent certain infections. Before joining, it’s important to understand that you must weigh at least 30 kg and have had CKD for at least 90 days. The study is not for those with recent infections, organ transplants, or those who've had specific treatments like plasmapheresis in the past year. Always discuss with a doctor to understand any risks and benefits.