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An axolotl dropping a leg isn't the end of the world. It's actually common when they've been injured in some way. Dropping the injured limb can actually help them regenerate better, so don't panic just yet.
First figure out why your axolotl would have dropped the leg, probably due to some kind of injury possibly from a tank mate. If your axolotl has a tank mate, I would separate them for the moment. If you have a plastic tub (like the size of a shoebox or something) I would move your injured axolotl to that for now, at least until you can get more information about this.
Let us see the wound, and we can go from there. It might be necessary to fridge your axolotl for a couple of days to make sure no fungus sets in, but let's wait to see how the wound actually looks.
In the mean time, make sure that the water conditions in your tank are good, and that the water is nice and cool. Even if you don't fridge your axolotl, the water will need to be clean and cold in the tank to make sure no infection sets in.
