We need some rash advice from nurse friends :)

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So...we know several of you have medical experience ... any suggestions for what this type of rash could be? Mason has it A LOT, and it usually tends to be after a surgery and shows up worst when he is crying but sticks around for a long time in a much more minor appearance - weeks on end). Could this be related to morphine or another anesthesia med? (we've read it has an allergic like reaction sometimes and he's had it mildly off and on since his previous MRI intubation). His upper airway also narrows and he sounds awful (hoarse, wheezy) but we've been told this was just from intubation irritation. If it's more likely that this is an allergic type reaction we need to do something to find out what is causing it so we watch out for it next time but...do we need a dermatologist to determine it, talk to anesthesia, or just who? Our pediatrician says it's probably "flushing" but seems pretty extreme if it's just that. It does look like blotches of flat blushing - also behaves exactly like hives (comes and goes at random places) but has absolutely no whealing (no raised areas at all). Anyone who can point us to someone who may be able to help us out, would be great. We'll try to plan a dermatology consult but you know by the time someone can see us, he won't be having the issue at the time. Thanks!!