The pediatric neurologist regularly deals with a wide range of serious childhood diseases as a doctor with the state-funded Children's Rehabilitative Services in Phoenix. Tarby says he quickly realized he was dealing with a very unusual condition that he could not diagnose.
He prepared urine samples and sent them to the University of Colorado Science Center's Dr. Steve Goodman, a professor of pediatrics who runs a laboratory that detects rare genetic diseases.
Goodman soon made a startling discovery: Tarby's young patient was afflicted with an extremely rare disease called fumarase deficiency.
"I had never seen a patient with it," Tarby says. "Right away I asked the parents if there were any other children with the same problem."
The parents said their daughter had cerebral palsy. Tarby asked them to bring the girl to him for an examination.
"As soon as I saw her, I knew she had the same thing as her brother," Tarby says.
The fact that fumarase deficiency had shown up in one child was startling enough -- there had only been a handful of cases reported worldwide. But now that it was appearing in two children in the same family was an indication it was being spread by a gene that was getting passed to the children by their parents.
Tarby and a team of doctors from Barrow Neurological Institute at St. Joseph's Hospital in Phoenix and the University of Arizona College of Medicine in Tucson began researching the disease and soon discovered that fumarase deficiency was occurring in at least two other families living in the same isolated community that practiced an unusual custom.
Nearly everyone in Colorado City, Arizona, and the adjacent town of Hildale, Utah, was a member of a fundamentalist Mormon sect that practices polygamy and had long encouraged multiple marriages between close relatives.
By the late 1990s, Tarby and his team had discovered fumarase deficiency was occurring in the greatest concentration in the world among the fundamentalist Mormon polygamists of northern Arizona and southern Utah.
Of even greater concern was the fact that the recessive gene that triggers the disease was rapidly spreading to thousands of individuals living in the community because of decades of inbreeding.
Fast-forward to the present: About half of the 8,000 people living in the towns are blood relatives of two of the founding families that settled in the 1930s on the desolate high desert plateau against the base of the Vermillion Cliffs.
Religious leaders control all marriages in the community, and many of these relatives have married or likely will marry in the future. Some of these marriages will include parents who both are carriers of the fumarase deficiency gene, making it certain that more children will be afflicted with the disease.
"We have and will have a continual output of children with this condition," Tarby says.
In this isolated religious society north of the Grand Canyon, few secrets have been more closely guarded than the presence of fumarase deficiency. Fundamentalist Church of Jesus Christ of Latter-day Saints elders, who control the community, have labored to keep the public from finding out why the disorder is manifesting. Many members of the fundamentalist community don't even know it's occurring.
The state of Arizona is contributing to the secrecy. The state Department of Health Services and the Department of Economic Security have been quietly providing services to assist the children and families of fumarase victims for more than 15 years. Both DHS and DES officials refused repeated requests from New Times to document the type and cost of services the state is providing to treat fumarase deficiency. The agencies claim that federal health laws prohibit them from releasing records or allowing their authorities to comment on the situation.
Doctors and family members interviewed by New Times say up to 20 children from families in the polygamist community are currently afflicted with the condition that requires full-time attention from caregivers. Victims suffer a range of symptoms, including severe epileptic seizures, inability to walk or even sit upright, severe speech impediments, failure to grow at a normal rate, and tragic physical deformities.
"They are in terrible shape," says Dr. Kirk A. Aleck, director of the Pediatric Neurogenetics Center at St. Joseph's Hospital. Aleck is a geneticist who participated along with Tarby and others in the groundbreaking study of several polygamous families with fumarase deficiency in the late 1990s.
There is no cure for the disease, which impedes the body's ability to process food at the cellular level.
"We can only treat the complications of the disorder," Aleck says. Once a baby is born with the condition, Aleck says, "You really can't treat the underlying disorder."