The most likely cause of Anna's poliovirus diagnosis is that she didn’t thoroughly wash her hands after using the restroom.
Poliovirus is primarily transmitted through the fecal-oral route, which means that it can spread when an uninfected person ingests contaminated food or water, or through direct contact with contaminated hands after using the restroom. This transmission route is much more relevant to poliovirus than the other options provided.
The other options listed are less relevant for poliovirus transmission: sharing needles is more associated with bloodborne viruses, consuming undercooked food primarily relates to bacterial infections, and inhaling respiratory droplets is not a known transmission method for poliovirus.