One prefers not to run into this while crawling around photographing wildflowers.
It can be identified by the three leaflets and the somewhat longer stem of the central leaflet.
Jewel Weed (see below in this gallery) relieves the symptoms.
The dreaded 3-leaf. What's so diabolical about this stuff is that it can get you even in winter when there are no leaves, or when burned. Lovely photograph of it, though. :)