STAR - Can you imagine donating an kidney to someone you don't even know? Inspired by his young son, Alex Fox did just that.

Alex's son, Karston, was born with a rare condition called Arthrogryposis, and is unable to walk. He also has trouble communicating, but the 6-year-old's zest for life is obvious.

"He's a very special, special little boy that brings a lot of joy to people," said Alex. "I've had people reach out to me and tell me how much he inspires them."

A year ago, Alex had no idea what was to come, or how his love for his son would save a life. In October 2015, he came across a Facebook post from a mother of a girl with special needs. Her daughter, 15-year-old Darienne Quiroz, of Utah, was in desperate need of a kidney, and her mom was pleading for a donor.

"I didn't know the mom at all, I didn't know the girl," explained Alex. "But I saw the picture and I just thought I needed to reach out to the mom.

"For me, what if that was Karston?" he added. "What if I needed him to get a kidney, and I wasn't a match."

Alex went through vigorous testing, and incredibly, was a perfect match. So earlier this year he traveled to Salt Lake City and donated one of his kidneys to Darienne, a girl he didn't even know.

"I was not fearful at all," he said. "I felt very calm, and I felt like this is what I was supposed to do."

The surgery was a success. Darienne's mom says there is no way to say thank you for such an extraordinary act of love, calling Alex their guardian angel.

"She's extremely grateful and says there is no way she could ever pay it back," said Alex. "But for me the payback is watching Darienne flourish. She's doing very well and that for me, is just great."

Alex says being a special needs dad has made him a better person, and it was Karston that inspired him to give such a special and selfless gift.

For more information on Alex Fox's story, click here. To learn more about kidney donation, click here.