The girl who doesn't grow: Meet Brooke Greenberg toddler-sized 20-year-old

Brooke Greenberg is a medical mystery that continues to baffle physicians.

At the age of 20, she weighs just 17 pounds and, at 30 inches tall, is the approximate size of a 3- or 4-year-old toddler.

Brooke just suddenly stopped growing at the age of four, her father, Howard Greenberg explained to the Daily Mail.

"From age one to four, Brooke changed," he said. "She got a little bit bigger. But age four, four to five, she stopped." Brooke has the mental age of about nine months to one year, and requires round-the-clock care from her parents. She eats through a tube inserted into her stomach, since her esophagus is too small to handle solid food, but she otherwise seems healthy.

The Greenbergs, who are from Maryland, paid a visit to Katie Couric's Katie, have taken Brooke to some of the most prestigious medical institutions looking for answers. But doctors have been unable to find an explanation, dubbing her condition 'Syndrome X.'

"She no apparent abnormalities in her endocrine system, no gross chromosomal abnormalities, or any of the other disruptions known to occur in humans that can cause developmental issues," explained Dr. Eric Schadt of the Icahn Institute for Genomics and Multiscale Biology at the Mount Sinai Medical Center in New York.

Schadt is in the process of sequencing her genome, with the hope that her genetic code will provide clues into how the human body ages. He speculates that Brooke may have a gene mutation that shuts off her ability to mature.

According to ABC News, Brooke struggled through a series of medical emergencies through her first six years, often recovering suddenly without explanation.

She suffered an apparent seizure, and was diagnosed with having a stroke, but recovered weeks later with no apparent damage, and at age 4, fell into a 14-day coma from which she awoke alert and unchanged.

Her mother, Melanie, says she sees adult emotions in her daughter others don't necessarily recognize.

"She loves to shop," Melanie said. "Just like a woman."

Pushing Brooke through the infant sections of stores, Melanie avoids telling people how old Brooke actually is.

"My system always has been to turn years into months," she said. "So, if someone asked today, I might say, she's 16 months old."