Avery Dox is just like a lot of 11-year-olds: she loves gymnastics and is excited for summer break. But she also happens to be Type 1 diabetic.

"I have to check my blood sugar about every three hours. Sometimes it's a little more, sometimes it's a little less," Dox said.

Since her diagnosis two years ago, insulin has become a critical part of her life. Her mother, Alissa Daire Nelson, said the family has insurance and didn't really feel the rising prices – until a recent night when she accidentally dropped and broke the last bottle. An emergency replacement supply cost $250.

"This is literally a matter of life or death. Avery has six or seven doses of insulin a day plus a long-acting one she needs to survive," Nelson said. "It was shocking to me because we had never been in that position that we had to pay out of pocket."

Nelson said she can only imagine the burden placed on people who have to pay full price.

"Why is it suddenly in the past 10 years the prices have skyrocketed to me? It just doesn't add up," she said.

Monday, Minnesota Democratic Sen. Amy Klobuchar sent letters to the heads of the top three producers of insulin -- Eli Lilly, Sanofi and Novo Nordisk -- asking them to justify the higher prices.

"And there's no arguable reason," Klobuchar said. "It's not like insulin has improved or there have been changes to it. They have simply jacked up the prices."

The companies have denied claims of collusion and said the higher prices are resulting from increased demand.

Klobuchar points to one example to illustrate the problem. She said the price of Eli Lilly's Humulin R U-500 insulin increased 325 percent between 2010 and 2015.

"This seems like abuse of monopoly power to me, and we've got to get to the bottom of it," Klobuchar said.

Added Nelson: "It really is just one of those things that's coming off the backs of people who don't have any other choice. And I just don't think that's very fair."

Klobuchar and fellow Democratic Sen. Al Franken have also called for easing restrictions on drug imports from Canada and faster approval of generics to improve competition in the prescription drug market.