Scratch my back – I've got an itch.

给我挠挠背–我痒痒。

《柯林斯英汉双解大词典》

Scratch yourself if you itch.

如果你感到痒，就搔好了。

《新英汉大辞典》

You may scratch an itch on your arm and kill lots of cells in the process without even thinking about it.

你可能在你的手臂上抓伤一英尺，可能在没想过的情况下已经杀死了很多的细胞。

youdao

Breaking this itch-scratch cycle can be challenging.

CNN: Atopic dermatitis (eczema)

"I'd like to think that I would do it just like I would scratch if I had an itch, " says Louis-Dreyfus.

FORBES: Why not a sonnet?

So if watching your fantasies play out on TV doesn't quite scratch that itch, you can enact them yourself down at the range -- or get a concealed carry permit, and convince yourself that the only reason you're not Jack Bauer is that the right opportunity hasn't yet presented itself.

CNN: Not man enough? Buy a gun