-- When Whitnie Strauss saw her toddler son almost drunkenly stumble around the backyard -- just the latest in a list of strange symptoms -- she said she realized she'd had enough.
"I just think it's so important not to give up, not to stop looking just because you've had door after door slamming in your face," she said.
So that day, she packed Reid up in the car and drove the more than two hours from outside Austin, Texas, to Houston. She said she knew she couldn't get an appointment at Texas Children's Hospital, but she could take him to the emergency room. And if they saw him, maybe they could solve the puzzle.
"This is drastic. You can say I'm 'that mom' if you want," Strauss said. "This is all I know to do."
They spent two "surreal" days at the hospital, where a team of more than a dozen doctors and medical fellows peppered them with questions, she said, adding that it reminded her of an episode of "House." Reid's ailment was still a mystery when they left, but one of the doctors told her that Dr. Michelle Holick, a pediatric neurologist there, wanted to solve the mystery.
"I wanted somebody to treat him as Reid and listen to his whole story, and they did," Strauss said. "They're not just going to stick a label on your child and send you on your way."
After several rounds of tests, Holick found an abnormality related to Reid's creatine, an acid the body makes to help provide energy to other cells, Holick told ABC News. Strauss did her own research and learned that of the three known creatine disorders, two were treatable.
But Strauss remembers the day Holick broke the news that Reid had the third kind: creatine transporter deficiency. Although creatine provides most of Reid's body with energy, he lacks the ability to get the energy-providing acid to his brain. And there's no treatment.
"You're hearing this terrible news, the worst news of my life," Strauss said. "But this weight is lifted because finally that burden of looking and trying to understand what's going on has been lifted."
Now 4 years old, Reid is a happy, "feisty" little boy, but he'll only have limited speech, his mother said. Still, he's learning to push buttons and point.
Creatine transporter deficiency is so rare that there isn't a lot of data on children older than Reid, but Holick said he's "leveling out."
"At this point, there's no expectation of going downward," she said, adding that Reid is still an active child who loves to run around the room.
Strauss said Reid understands instructions, but that doesn't mean he'll listen.
"Sometimes, he'll lay down on the floor and just giggle, 'No, sorry. Not gonna do that today,'" Strauss said. "He tries to be the boss of you."
Reid tries to avoid clothes whenever he can, so Strauss sewed special buttons to hold his shirts and pants together to keep him from disrobing.
The Strauss family is also trying to incorporate picture cards into his routine to help him communicate.
"Listen to your instincts and that gut feeling. Parents know," she said. "For us, it took 20 different doctors before we stumbled upon the one at Texas Children's, who for whatever reason dug deeper than all the others. ... You never know when you're going to open the door and your Dr. Holick will be standing there."