The family of 3-year-old Amelia Rivera has known since her birth that the brown-eyed girl was headed for kidney trouble.

And sure enough in December, a Children's Hospital of Philadelphia physician said the girl would need a transplant within six months to a year, according to her mother, Chrissy Rivera.

For now, however, it won't be happening at Children's.

The girls' parents say they were told last week by a different physician at the hospital that she was not a good candidate for a transplant because she was "mentally retarded." Amelia, who goes by the nickname Mia, has Wolf-Hirschhorn syndrome - a rare genetic condition that results in substantial developmental delays and a shortened life expectancy.

Hospital officials say they cannot comment on any individual cases but add that the institution "does not disqualify potential transplant candidates on the basis of intellectual abilities."

The case has drawn national attention, including at the website, where a petition urging the hospital to perform the transplant had drawn more than 22,800 signatures by late Tuesday.

"It's been extraordinary," Chrissy Rivera said of the support.

Amelia is far from the first disabled person who has had difficulty getting an organ transplant. People with Down syndrome, for example, are often steered away from heart transplants, on the theory that they may have difficulty following the complex postsurgery drug regimen. Hospitals also commonly take into account other factors in deciding against a transplant, such as advanced age.

Yet denial of a transplant purely on the basis of a disability would violate the Americans With Disabilities Act, said Lara Schwartz, vice president of external affairs at the American Association of People With Disabilities.

"Such a case has never been challenged under the ADA, but I think it might make a very strong case," said Michele Goodwin, a University of Minnesota law and medicine professor who researches organ transplants.

Amelia's case began to attract attention after her mother wrote an emotional blog post on a site for families affected by Wolf-Hirschhorn syndrome, describing her meeting with a hospital physician and a social worker. Chrissy Rivera and her husband, Joe, of Stratford, Camden County, had gone to the hospital last week to ask how family members could get tested to see if any of them were suitable kidney donors.

Instead, the girl's mother said, the physician told her the hospital would not perform the surgery. She said there was no explanation of why the girl's disability might make her medically unsuitable for a transplant.

But the social worker raised the issue of who would help Amelia take her medication once her parents were no longer around, said the girl's mother, 36.

In a telephone interview, Chrissy Rivera criticized that reasoning, saying that whatever may happen years from now is no reason to deny her daughter a lifesaving surgery. Some people with Wolf-Hirschhorn syndrome live for just a few years, while others have lived into their 40s. Meanwhile, kidney transplants from a living donor last for 15 to 18 years on average, according to the American Society of Transplantation.

"She takes medicine now and we make sure that she gets it," Chrissy Rivera said. "Of course we're going to follow her care."

She said the family had medical insurance but had not yet looked into how much of a transplant procedure would be covered.

Hospitals have a fair amount of leeway in determining which patients they will recommend for a transplant, said Robert Gaston, president of the American Society of Transplantation.

Under federal regulations, they are required to specify their own criteria for transplant approval and to follow the policy consistently, said Gaston, medical director of the adult kidney and transplant program at the University of Alabama at Birmingham Hospital.

He said cognitive deficits were typically taken into account, but they do not spell automatic rejection.

"In a person who has insufficient cognitive skills to be able to follow a transplant regimen, then the requirement is that there is a caregiver who can overcome that deficit," Gaston said.

He said he could not comment on the Rivera case because he did not know the details, but pointed out that if patients were denied at one hospital, they could always try another.

But the Riveras would rather not go elsewhere if they don't have to.

Children's Hospital has contacted Amelia's parents to say they are welcome to come in for another meeting, Chrissy Rivera said.

"We've had great experiences with CHOP," Chrissy Rivera said. "That would always be our first option."