Kristen and Clayton Greene gave birth to a healthy baby daughter but at 15 months old, Cara lost her ability to walk, her arms became limp and her grip was too weak to hold on to anything.
"We had no idea what was wrong," Kristen, 32, from Wilmington, North Carolina, tells PEOPLE. "It was horrific."
Medical experts had trouble diagnosing Cara, and each trip to the doctor was more confusing than the last.
"She was just getting sicker and sicker," she says. "My husband was holding her and he remembers thinking that one day she'll be in his arms and she won't be alive."
The devoted parents never gave up hope and decided to give precision medicine a shot with the help one of determined doctor.
Dr. David Goldstein, director of the Institute of Genomic Medicine at Columbia University, was working on gene sequencing and took on Cara's case.
"We got this phone call that they had the results, but we had to wait an entire weekend to find out what they were," says Kristen. "It was torture. We thought they were going to say she was going to die, so we spent the weekend grieving."
Dr. Goldstein told them that they not only had an answer to her health's steep decline – they also had a simple cure.
"He told us they can rarely give parents good news," she says. "And on top of that, we could help her."
Doctors discovered from the genetic sequencing that Kristen and Clayton, 31, both carried a gene mutation that affected Cara's ability to produce a protein, resulting in a severe vitamin B2 deficiency.
Her condition, Brown-Vialetto-Van Laere syndrome, a rare, neurodegenerative motor neuron disorder, only has approximately 70 known cases worldwide.
Their story is part of WebMD's new video series on cutting-edge medical innovation, Path to a Breakthrough, produced by Good Morning America's Robin Roberts.
With a simple treatment of a high dose of vitamin B12 in the form of an oral supplement, Cara, who turns 4 on August 18, quickly began to improve.
"We are so lucky," says Kristen. "It makes you realize a lot. We celebrate milestones that other families might not because we know how precious life is."
Dr. Hansa Bhargava, medical editor and WebMD's expert pediatrician, tells PEOPLE that a full genetic analysis was very expensive a few years ago, but it now costs significantly less and will only continue to become more affordable.