This clinical trial is focused on studying the effects of a medication called Guselkumab in children and teenagers with a condition known as Ulcerative Colitis. Ulcerative Colitis is a disease that causes inflammation and sores in the lining of the large intestine, which can lead to symptoms like abdominal pain and diarrhea. The medication being tested, Guselkumab, is a type of treatment known as a monoclonal antibody. Monoclonal antibodies are proteins made in a lab that can help the immune system fight diseases.

The purpose of this study is to evaluate how effective and safe Guselkumab is for young patients with moderately to severely active Ulcerative Colitis. The study will involve an initial phase where participants receive the treatment openly, followed by a phase where neither the participants nor the researchers know who is receiving Guselkumab and who is receiving a placebo. This helps ensure that the results are not biased. The study will last for a period of time, during which participants will receive the treatment and be monitored for any changes in their condition.

Participants in the study will receive Guselkumab through injections, either under the skin or into a vein, depending on the specific formulation used. The study aims to see if Guselkumab can help achieve clinical remission, which means reducing the symptoms of Ulcerative Colitis to a minimal level. The trial will also look at the safety of the medication and how it is processed in the body. This research is important for understanding if Guselkumab can be a beneficial treatment option for young people with this challenging condition.