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When Will Schuller was an 18-year-old senior in high school in Overland Park, Kansas, something puzzling was happening. He was “big into running at the time” and seemingly in good health, but his mile time kept increasing.

“I was getting much slower and slower times,” Will, who is a now a senior at the University of Tulsa studying mechanical engineering, told NBC News on Thursday. Eventually, he said, it got the point that within weeks he "couldn't run even half the distance."

From there, it only got worse. Will struggled to walk from the school parking lot to his classes, and he couldn’t exercise.

“Everyone knew I was this fit guy, and in just a matter of a month you see me struggling to walk from one side of the hallway to the other,” he said. “My sister would have to piggyback me up the stairs.” At that point, Will's parents had to pull him out of school.

It took about six weeks for a neurologist to figure out what was going on. Right around Christmas 2014, Will was diagnosed with Lambert-Eaton Myasthenic Syndrome (LEMS), a rare neuromuscular disorder.

LEMS is “a chronic autoimmune disorder that affects strength and muscle fatigue ability,” said Dr. Ted M. Burns, a professor of neurology at the University of Virginia.

LEMS affects about 3,000 people in the U.S., Burns said, and it can drastically alter the quality of their life.

There is a drug to treat LEMS that up until recently was free for patients through an FDA program called “compassionate use.”

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Will Schuller, who has Lambert-Eaton Myasthenic Syndrome.

When Schuller got the drug, called 3,4-diaminopyridine, or 3,4-DAP, he instantly felt better.

Ann Schuller, Will’s mother, said that about 40 minutes after her son took the pill for the first time, he was up and celebrating at the Mayo Clinic in Minnesota, where he was being treated.

“He skips out to the lobby. And we skip on down to the next appointment. And I'm with the wheelchair trying to follow him back down,” she said. “I don't believe in miracles. But that was pretty darn close to one. ... It was a miracle drug, for sure.”

Since 1992, 3,4-DAP, was made by Jacobus Pharmaceuticals, a small New Jersey company, until a different company, Catalyst, recently received the exclusive rights to the drug. Catalyst added a preservative, renamed it Firdapse, and is now charging north of $375,000 a year for the life-changing drug.

“This is not a story about innovation. This is a story about exploitation,” Burns said.

Will's parents said doctors at the Mayo Clinic warned them about an impending price increase, but they never imagined it would start costing hundreds of thousands of dollars.

“We're paying something dramatically less than that,” said Bob Schuller, Will’s dad. “But everyone's premiums are going to go up as a result of this. So it's a cost to the entire system.”

Sen. Bernie Sanders, I-Vt., sent a letter to Catalyst on Monday demanding an explanation for the new price of Firdapse, calling it "a blatant fleecing of American taxpayers."

Firdapse, tablets for the treatment of Lambert-Eaton myasthenic syndrome (LEMS) in adults.

However, Catalyst says the cost increase is both legal and reasonable.

"Our pricing strategy reflects our focus on ensuring broad and sustainable coverage from both private and public payers and assistance for patients in need," Catalyst CEO Patrick J. McEnany told investors in a conference call in December.

Catalyst told NBC News that "most patients will pay approximately $10 out of pocket as a co-pay" for Firdapse each month and said it "will respond to Senator Sanders’ letter in a timely manner."

Burns says that while it's true patients won't be bearing the brunt of the price increase, Catalyst bought the drug "knowing that they could charge whatever they wanted if they got FDA approval."

“They could charge $10 million and I would have to find a way to pay for it,” Will said. “I don’t think I would be alive today without DAP. I would for sure be in a wheelchair."

His mother has a question for Catalyst: "Why is my insurance company having to pay that much money for something that doesn't cost that much to make?"

“Is that really the kind of system we want to have?” Ann Schuller asked.