歯が___ので、歯医者に行きます。
Q. Which word naturally completes 'my tooth hurts'?
Translation
My tooth hurts, so I'm going to the dentist.
Explanation
痛い is the adjective 'painful': 歯が痛い. 怪我する is for injuries from accidents, and 病気 is an illness — neither collocates with a toothache.