癣(xuǎn) - ringworm, tinea in Mandarin

癣 (xuǎn) is the fungal skin infection English calls ringworm. It names the whole family: 脚癣 is athlete's foot, 头癣 is scalp ringworm, 体癣 is the ring on the body, and 牛皮癣, ox-hide ringworm, is the everyday name for psoriasis.

It is a chemist's word rather than a rude one, so it is perfectly comfortable to say across a counter. 长癣 is to develop it, 治癣 is to treat it, and the thing you buy is 药膏, an ointment. City-dwellers also use 牛皮癣 figuratively for the small fly-posted adverts stuck all over walls and lamp posts, which is a rather good image.

Two traps. Despite the English name there is no worm involved at all, so do not go looking for 虫 in the vocabulary around it. And the syllable is crowded: 选 xuǎn, to choose, is one of the commonest verbs in the language, so a bare 癣 heard out of context will almost always be misread as 选 unless the topic is already the skin.