It's a rule that Janet -- who's been dealing with getting her daughter a transplant for 18 months and who "did her research" -- learned about just two weeks ago.
"I was shocked," Janet said.
Dr. Stuart Sweet of St. Louis Children's Hospital is a board member at the United Network for Organ Sharing and helped write the current pediatric transplant system.
"It's not a perfect system. There is no perfect system," Sweet said in an interview. "It's the best we can do right now."
Sweet said Sarah's story "tugs at his heart" but that if he changed the system for Sarah's advantage, "there's another patient, very likely an adolescent, who gets a disadvantage."
"We've built a system that tries to be as fair to everyone as possible."
Mom and dad know that the rules are making it almost impossible for their daughter to receive a pair of lungs through the transplant system. But there is a shred of hope: There is always the possibility that someone steps forward with a direct donation to Sarah.
Children's Hospital of Philadelphia declined an interview with CNN for this story, but Janet said doctors are telling her that Sarah could "crash" any time now and need intubation, the insertion of a tube into her lungs for breathing assistance. The procedure is especially dangerous for cystic fibrosis patients.
She said doctors are planning to operate in coming days in an effort to divert that crash, which should buy her a few more weeks and more time for a potential direct donor to reach out to the Murnaghans. This procedure is known as an ECMO -- extracorporeal membrane oxygenation -- and is used occasionally as a bridge to lung transplantation.
For now, Sarah, who has been outdoors only twice in the past 100 days, is looking forward to a possible transplant. As she puts it, "I'm not going for easy -- I'm just going for possible."
"We will (get them)! I can't wait to take my first breath with new lungs. I can close my eyes right now and imagine it."
"Are we gonna quit?" mom asks.
Sarah deliberates very briefly.
"No!" she shouts at the top of her lungs and slapping the bed with both hands. "I'm never going to quit! Never, never!"