Blood-red Bone-hard Blisters Kid's Story

Hi. Looking for a story my sister read as kid on a reading comprehension card in the 70's. You know the ones with levels in colors, red, blue, green, goldenrod etc. She says it was great little scary short story with punchline about blood-red bone-hard blisters and of course has not seen it anywhere since, doesn't know the author's name. Any help finding this would be appreciated. Thanks ever so.