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MacKenzie Maddry always loved sports. The eighth grader ran track and cross country, and she also played soccer. So when she told her parents her left leg hurt in fall 2020, they thought it was a sports-related injury.

“I noticed this bump on my leg, and it just got bigger and more painful,” the teen from Bella Vista, Ark., tells PEOPLE. “We just thought I had pulled something.”

The lump was on the inside of her left thigh, just above her knee. And when she sat down — to play the viola for example — it became more painful.

“I would go to orchestra and sit down, and then it would take me a while to get up and be able to straighten my leg out. It was so bad,” MacKenzie, now 17, says. “That was when I was like, ‘Yeah, we definitely need to see a doctor about this.’”

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