PHILADELPHIA — A 10-year-old Pennsylvania girl who had two adult-lung transplants after her parents sued to change national rules regarding organ donations has developed pneumonia in her right lung.

Sarah Murnaghan’s mother wrote on her Facebook page Monday that Sunday was “a tough day” but Sarah’s condition has stabilized.

Still, Janet Murnaghan calls the infection a large setback for her daughter.

Sarah has been at Children’s Hospital of Philadelphia for months with end-stage cystic fibrosis. The Newtown girl was a top candidate for organs from a child donor but none was available, and a national transplant policy put her at the bottom of the adult list.

Her parents then sued to change the policy. The action spurred a national debate and raised questions among some health experts and medical ethicists.