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