一颗老鼠屎坏了一锅汤

yī kē lǎo shǔ shǐ huài le yī guō tāngyikelaoshushihuaileyiguotang

1. lit. one pellet of rat feces spoiled the whole pot of soup (idiom)
2. fig. one bad apple can spoil the whole bunch

Study it in Yanamino