Watashi no Hada ni Noroi no Kao ga

A grotesque, horrifying, mysterious incident befalls 12-year-old Yumi. At Yumi's birthday party, which she invites classmates to, her mischievous older sister Mari puts a toy toad's leg in the food. Sayuri, who has an abnormal hatred for frogs, ends up eating it. However, when she spat it out, the frog's leg was still slithering about as if it was alive. Sayuri stayed home from school after that, but terrifying changes began to occur to her body...

(Source: K-Manga)