Yesterday, photos of a woman with 10 fingers and no thumbs surfaced on Reddit, leading users of the news aggregator and Gawker to collectively ask “WTF?”

As some medical students ultimately pointed out, the woman was likely born with triphalangeal thumb, a condition where an extra finger bone — a phalange — grows in the hand’s most useful digit.

Approximately one in 25,000 children is born with a triphalangeal thumb, which manifests when an extra phalange grows during embryonic development and extends the length of the thumb. The longer digit can sometimes function like a normal thumb. Alternatively, it can be shaped like the other fingers of the hand, such was the case of the Reddit user’s co-worker.

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Reddit Reddit

This condition is often seen with ‘polydactyly’ fingers, where an extra finger grows on top of or out of another one, typically the thumb.

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Polydactyly thumb. Credit: Humpath.com Humpath.com

A triphalangeal thumb can be treated with reconstructive surgery that reshapes the finger, by adding ligaments and tendons, so it operates like a thumb. Some cases are hereditary, and the condtion is a feature of rare disorders like Holt–Oram syndrome, Aase syndrome, and Diamond–Blackfan anemia.