This study is about testing a medicine called ravulizumab for people with a condition called HSCT-TMA, which can happen after a bone marrow transplant. HSCT-TMA stands for Hematopoietic Stem Cell Transplant-associated Thrombotic Microangiopathy. In simple terms, this condition involves small blood clots forming in the blood vessels after a bone marrow transplant. The study has two parts. In the first part, everyone gets the medicine to help decide the right dose. In the second part, some people get the real medicine, and some get a fake one (placebo) to see if it really works. The treatment lasts for 26 weeks, followed by another 26 weeks of checking up on the participants. To join, you must be at least 12 years old, have had a stem cell transplant in the last year, and meet other health criteria. You can't join if you have certain other health problems or are pregnant.

• The study lasts for about a year (52 weeks total).
• Participants will receive either the medicine or a placebo.
• There are specific health conditions that could exclude you from participating.