Just thought you might like to know how the vet trip went. The vet said that the most likely cause of the bald patch, especially considering its location and Quigley’s young age, is mites. Another possibility would have been ringworm but that usually causes very dry flaky skin and Quigley’s skin is not dry.
The vet gave Quigley an injection of mite treatment and in two weeks I need to give him three drops of liquid mite treatment on the back of his neck (this dose is calculated according to weight so will vary between hamsters).
Since Yamagi’s symptoms are similar to Quigley’s, I’d say the problem is most likely mites or ring worm if the skin is very dry and flaky.
Either way, the best thing to do will be to take him to the vet to get him checked out and treated. Ideally, you want to go to an exotics vet (hamsters count as exotic animals strangely!).
Good luck
let us know how everything goes.