Never had poison sumac but I am having a terrible itchy problem myself- TMI, but I have psoriasis and have had it since i was little- on my scalp only. I told my Doc that I have been really itchy and dry in my nether regions and that it had cracked open and bled even. She asked if it was psoriasis (she knows i have it on my scalp) and i said i didn't think so, i just thought it was my sensitive skin getting dry and itchy.  Well, she recommened hydrocortizone cream 3 times a day and it's helping, though i find I have to shave EVERY day down there. If i get the least amount of stubble it irritates it worse. And, now that I've been paying close attention, it IS scaly and patchy like psoriasis..so it looks like I have psoriasis in my nether regions- yuck. I hope your poison sumac isn't "down there" too, but if so I feel for you! I was reading the hydrocortizone tube and it says one of it's uses is poison sumac- I'd call your doc to get the go ahead to use it- it helps me lots. (or whatever else your doc recommends)