Jan. 22, 2007  -- Nine-year-old Kay Salomon is the picture of health. She enjoys many activities like skateboarding, surfing and swimming.

"I am kind of a big athlete," she says.

But Kay's mother, Kerry McAuley, lives in constant fear. This fear isn't just the worry of a normal parent, and McAuley's not just afraid that Kay will get hurt. She's constantly worried that Kay might die.

"I go in there many times during the night and check on her and make sure she's okay, wondering if this is the year or is it going to be in six months… when is she going to get sick on me?" says McAuley. "Because it could happen … it might not happen, but nobody knows."

Watch the story on "Primetime: Medical Mysteries" Wednesday at 10 p.m. ET, and go to ABCNEWS.com during the show to solve a real medical mystery.

Reversed Organs

To understand McAuley's concerns, you have to understand what's happening inside her daughter. And we're not talking about getting inside Kay's head or understanding her feelings. We mean literally looking at her internal organs, which are all on the wrong side of her body.

That's right, her stomach, liver, spleen and the rest of her organs are all reversed. Kay's insides are the mirror image of a normal child's. It's a condition called situs inversus, and though rare, it's probably more common than you think, affecting about one in 10,000 people.

Situs inversus isn't necessarily a cause for alarm. Many people with the condition are actually perfectly healthy. But Kay has an added complication: corrected transposition of the great arteries. This means that parts of her heart are switched around as well. In her case, it's the upper chambers, or atria, and her great arteries. Amazingly, the blood still gets to the right place, just through the wrong connections.

"The easy way to look at that is two wrongs make a right," says Dr. Frank Hanley, a pediatric cardiac surgeon at Packard Children's hospital near San Francisco.

But Dr. Hanley says the odd structure of Kay's heart is not the main concern. Kay's biggest problem is a large hole between the lower chambers of her heart, or ventricles, and an obstruction under one of her valves.

"Most of the time, if you have only one of those you have to have surgery relatively early in life," says Dr. Hanley.

Defying Statistics

Amazingly, so far Kay's system is working just fine.

"Her circulation is perfectly balanced," Dr. Hanley points out.

The question is this: How long will Kay stay healthy? Specialists have told her mom that people with similar defects often wear out their hearts and can die before reaching 40.

"Here's a little girl who's perfectly fine right now," Dr. Hanley says. "But you could make the argument that there's never a better time to do the procedure than now, because this is when her heart is the healthiest. Whether we can wait five years, ten years and get the same result with the same low risk -- it's hard to say."

So here's the tough decision Kerry McAuley is facing: Should she let doctors perform a dangerous open-heart procedure on her daughter who, today, looks perfectly healthy, or wait, possibly forcing the heart to exhaust its smaller, weaker valve? And waiting could make an operation even more difficult, maybe impossible.

Assessing the Situation

"It's the worst decision possibly for me to try to make. And it's causing me such stress," McAuley says. "I sleep with it, I wake up with nightmares, anxiety attacks, thinking am I doing the right thing leaving her alone? I'm going in every direction I can before I have to do the surgery."

Twice a year, McAuley takes her daughter to see top cardiologist Dr. Ira Parness at Mount Sinai hospital in New York City. Dr. Parness carefully examines Kay's heart and helps her mom assess the risks of doing nothing versus undergoing open-heart surgery.

"Big operation … long. Let's say three out of 100 patients in Kay's position would die," Dr. Parness explains. "So, I always tell patients that statistics lie. You either escape complication or not. So now, the question is, are you so convinced you need to do this that in the awful, unlikely possibility that she doesn't make it through and she dies, that you can live with that decision?"

A Second Scenario

Tom and Tonya Preston were also faced with this heartwrenching dilemma. Their daughter Carly was also born with corrected transposition of the great arteries with similar defects. Like Kay, she appeared perfectly healthy. However, unlike Kay, the rest of Carly's organs were in their proper places.

"Initially, they told us she would probably need surgery at 4," says her father, Tom.

So as Carly grew, the Prestons not only got a second opinion, they got a third, and a fourth. And the decision became even more complicated.

"They all wanted to do something different," Tonya Preston said. "So now, you're left weighing not only do we do surgery or not do surgery, but if we do surgery, which one?"

Doctors began to notice that Carly's oxygen level was slowly dropping each year. They told the Prestons it was time for open-heart surgery.

"And it sounds strange," says Tonya Preston, "but I was elated, because I was so tired of thinking about it and not knowing what to do."

The Prestons followed the doctors' advice and Carly went through open-heart surgery at age 7. To find out how Carly fared in surgery, watch "Primetime: Medical Mysteries."

Wait and See

Since Kay's heart is still working quite well, McAuley has decided not to proceed with an operation… right now. But she is still consumed with indecision because she knows her daughter's chances of a good recovery are much better while she's young.

Meanwhile, McAuley has become a CPR instructor and advocate. She also tries to use her skills and expertise to help raise awareness about congenital heart defects and plans to begin a fund for Kay's future operation.

About the Surgery

Carly Preston's surgery lasted about five hours. During that time, a specialist was able to re-reroute her blood flow to mimic a normal heart. And a bypass machine took over pumping her blood while her problematic valve was replaced with an artificial one.

That was about three years ago. Today, Carly is as active as most other 10-year-olds. She runs, jumps and performs ballet. But she still remembers waking up after the surgery.

"I was sore," Carly says. "And when I saw my stitches and stuff, I was kind of like, wow. I couldn't walk for a long time because all my muscles were used to lying down."

Since her doctor had to replace one of her valves with an artificial one, Carly will have to undergo more surgeries as she gets older.

"Future operations are required because children outgrow valves that are put in, just like they outgrow their little t-shirts and shoes," Dr. Hanley says.

But with modern medicine on her side, Carly's now likely to live a long and healthy life.

For more information about this story:

Corrected Transposition of the Great Arteries

Lucile Packard Children's Hospital

The Children's Heart Fund