This study aims to test how well and safely KarXT works for treating mania in people with Bipolar-I Disorder. Bipolar-I Disorder is a mental health condition that causes extreme mood swings, including emotional highs (mania) and lows (depression). In this study, participants must already be taking one of three mood stabilizers: lithium, valproate, or lamotrigine. These medications help balance mood. **Key Points:** - **Duration:** The study will require hospitalization due to acute mania, but the exact length of the study is not specified. - **Participation:** Participants must have had stable doses of mood stabilizers for at least two weeks and meet several health criteria. - **Risks:** People with other serious mental health issues or substance use disorders, and those with certain medical conditions like liver problems or glaucoma, cannot join. The study involves checking if KarXT can help manage mania better when used with existing medications. Participants will be screened to ensure they meet the criteria and do not have disqualifying health conditions.