Froggie Defect/Problem

[1beataway]

So, I'm watching my friend's golden tree frog for a year, and I remember she adopted this frog because it had a problem, and we can only sell healthy pets. I've tried and tried and tried to remember what the problem is called, but I simply cannot. Who can tell me? It's the "bump" on its back. What is it, and what causes it? Thanks!