This study is researching an investigational drug, ALN-HSD called "study drug". This study is focused on participants who are known to have metabolic dysfunction-associated steatohepatitis (MASH). MASH is a form of metabolic dysfunction-associated steatotic liver disease (MASLD). MASH occurs when fat builds up in liver cells, damaging them, and making the liver inflamed and stiff from fibrosis (scar tissue). MASH can progress to cirrhosis (long term scarring) and liver failure (when the liver cannot perform its job). The aim of the study is to see the effect of the study drug on lessening liver scarring side effects related to MASH. The study is looking at several other research questions, including: * How ALN-HSD works to improve liver function and lessen MASH-related inflammation in the liver * What side effects may happen from receiving the study drug * How much study drug and study drug metabolites (byproduct of the body breaking down the study drug) are in the blood at different times * Better understanding of the study drug and MASH