A woman charged in the fatal beating of her 3-year-old son has pleaded not guilty to murder. Jessica Trefethen, of Stockton Springs, entered the plea Monday afternoon, about four months after her son, Maddox Williams, arrived at the hospital. The medical examiner said Maddox died from multiple blunt-force injuries including a fractured spine, a brain bleed and ruptured bowel. Maddox was one of four children who died in June, and his death attracted widespread attention in the state.