Toxicodendron vernix

Grows in swampy places with 7 to 13 leaflets, so the leaves-of-three rhyme will not save you here.
It causes the worst rash of the three by a long way. This is the one to actually memorize.
This one is poisonous, which means it hurts you if you touch it or eat it. It cannot chase you, so the rule is simple: look, do not touch.
Always ask a grown-up before you touch any animal or plant. If something bites or stings you, tell an adult right away.
Source: our source list
Read more about the poison sumac at Wikipedia.