Translation:Go buy something for toothaches.
"Go buy something for your toothache." Rejected. Duo has long regarded "le/la" [part of body] to mean "your/my." Why not this time? Duo's own hint here includes "your." It certainly seems far more likely that this is what one would say to a toothache sufferer than Duo's solution.
OK, lots of aches about forms of buying going on here! However what is actually going to be bought is something that either "cures" toothache (eg antibiotics) or takes away the pain of toothache (ibuprofen etc) . In common speech this gets abbreviated in English to "for" - and in French to "contre" - It just goes to prove that the English and the French can always be relied upon to take the opposite view of the same thing!