(CNN) On an early spring day in 2018, the faint smell of formaldehyde floating in the air, 26-year-old medical student Warren Nielsen and four of his classmates prepped a cadaver in the chilly dissection lab at Oregon Health and Science University in Portland.

Similar groups of five gathered around bodies on the other 15 tables in the anatomy class, all eager to explore the mysteries of the human body they had seen only in textbooks.

The cadaver assigned to Nielsen's team was a 99-year-old woman who had died of natural causes. Her name was Rose Marie Bentley, but the students didn't know that then. To honor and respect the privacy of those who offer their bodies to science, no further details are given medical students about the person who had once inhabited the body lying on the silvery slab before them.

Rose Marie Bentley, not long before she died and donated her body to science.

But as the students and their professors were soon to find out, Bentley was special, so special she deserved her own unique spot in medical literature and history books.

The reason? A condition called situs inversus with levocardia, in which most vital organs are reversed -- almost like a mirror inside the body. That, along with a host of other weird but wonderful abnormalities, made Bentley a sort of medical unicorn.

"I think the odds of finding another person like her may be as remote as one in 50 million," said assistant professor Cameron Walker, who teaches the Foundations of Clinical Anatomy class at Oregon Health and Science University. "I don't think any of us will ever forget it, honestly."

'This is totally backwards'

On this March day, the assignment was to open the body's chest cavity to examine the heart. It wasn't long before Nielsen's group began to question their fledgling medical knowledge.

"Her heart was missing a large vein that's normally on the right side," Nielsen said.

Bewildered, he and his team called the professors over and asked: "Where's the inferior vena cava? Are we missing it? Are we crazy?"

"And they kind of rolled their eyes," Nielsen said, "Like, 'how can these students miss this big vessel?' And they come over and that's when the hubbub starts. They're like 'Oh, my God, this is totally backwards!' "

A typical body has a large vein called the vena cava that follows the right side of the vertebral column, curving under the liver and emptying deoxygenated blood into the heart.

Bentley's vein was on the left, and instead of terminating directly into the heart, which is typical, "her vein continued through her diaphragm, along the thoracic vertebrae, up and around and over the aortic arch and then emptied into the right side of her heart," Walker said.

"Normally speaking, none of us have a vessel that does that directly," he added.

That wasn't the only irregularity Walker and his students found in Bentley's body.

Numerous veins that typically drain the liver and other parts of the chest cavity were either missing or sprouting from an unusual spot. Her right lung had only two lobes, instead of the standard three, while the right atrium of her heart was twice normal size.

"And instead of having a stomach on the left, which is normal, her stomach was on the right," Walker said. "Her liver, which normally occurs predominantly on the right, was predominantly on the left. Her spleen was on the right side instead of its normal occurrence on the left. And then the rest of her digestive tract, the ascending colon, was inverted as well."

The mutations in situs inversus with levocardia occur early, Walker explained, possibly between 30 and 45 days into the pregnancy. No one knows why.

The condition occurs in only 1 out of 22,000 babies and is invariably associated with severe congenital heart disease. Because of the heart defects, only 5% to 13% live past the age of 5; case reports mention one 13-year-old boy and a 73-year-old who at the time was the second-longest survivor.

But Bentley was an anomaly, one of the few born with the condition that didn't have heart defects, Walker said.

"That is almost certainly the factor that contributed most to her long life," he said.

And that, along with all her other exceedingly rare anatomical abnormalities, is what makes Bentley 1 in 50 million, Walker estimated.

'Mom would have been so tickled'

Rose Marie Phelps was born in 1918 in Waldport, a small town on the Oregon coast. The youngest child of four, "she was babied," said daughter Patti Helmig, who at 78 is the oldest of her five children. "She would admit she was spoiled."

A hairdresser by trade, Bentley was always fascinated by science, Helmig remembered, and she believes her mother would have made a fine nurse if she had been given the opportunity to train.

"She volunteered during World War II for one of the nurse's aid corps," Helmig said. "And she was thrilled when someone reached out to her about doing a study on smallpox survivors, which she had as a child."

Rose Marie Bentley with a big catch.

Despite chronic heartburn (which would have been explained by her unusual gastric anatomy), Bentley never showed any negative effects from her flip-flopped innards, said 76-year-old Ginger Robbins, the third of Bentley's children.

"We had no reason to believe there was anything like that wrong," Robbins said. "She was always very healthy. She was always doing something, taking us to Campfire Girls, fishing, swimming. She was an excellent swimmer."

The only clue anything might be unusual came when Bentley's appendix was removed, said 66-year-old Louise Allee, the fourth-born child and youngest of the daughters.

"The surgeon made a note that her appendix wasn't in the right spot when they took it out," Allee said, "but never said anything to us. Nobody said a thing when they took her gallbladder out and did a hysterectomy, either."

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The decision to become a body donor began with Jim Bentley, Rose Marie's husband, but she too "thought it was the greatest thing," Allee remembers.

"There was a poem that my dad found, and it was all about donating your parts," she said. "You know, 'give my eyes to a man who has never seen the sunrise' and the like. He kept showing us the poem. It was really important to them."

The poem, written by Robert Test, opens with the line: "Give my sight to the man who has never seen a sunrise, a baby's face, or love in the eyes of a woman," and ends with "If, by chance, you wish to remember me, do it with a kind deed or word to someone who needs you. If you do all I have asked, I will live forever."

The couple's beliefs about donation made an impact. All three daughters plan to donate their bodies for research.

Rose Marie Bentley, a one in 50 million gal.

Jim Bentley kept his promise and donated his body when he died of pneumonia more than a dozen years before his wife's death. His daughters know that he would have loved to have known about his wife's peculiar insides so he could have teased her about it.

"He also would have been tickled they could teach medical students something so different and really make some great use of her body," Allee said.

And what would Bentley have said about being a one in 50 million kind of gal?

"She would've just thought it was funny," Robbins said.

Allee agreed: "She would have had a big smile on her face."