Complex sentence: The named cholinergic urticaria comes from studies where some people with the disorder produce a rash when injected with the neurochemical acetylcholine; however, it is not clear how acetylcholine is involved in the reaction.
Split into simpler sentences: The named cholinergic urticaria comes from studies where some people with the disorder produce a rash when injected with the neurochemical acetylcholine. Though it is not clear how acetylcholine is involved in the reaction.