nettle rash vs skin rash

nettle rash and skin rash both are nouns.

Word	Noun	Adjective	Verb	Adverb
nettle rash	Yes	No	No	No
skin rash	Yes	No	No	No

As nouns, skin rash is a hypernym of nettle rash; that is, skin rash is a word with a broader meaning than nettle rash:

nettle rash: an itchy skin eruption characterized by weals with pale interiors and well-defined red margins; usually the result of an allergic response to insect bites or food or drugs
skin rash: any red eruption of the skin

Other hypernyms of nettle rash include efflorescence, rash, roseola, hypersensitivity reaction.

nettle rash (noun)	skin rash (noun)
an itchy skin eruption characterized by weals with pale interiors and well-defined red margins; usually the result of an allergic response to insect bites or food or drugs	any red eruption of the skin