固执，认死理single-minded

• 她是一个特别单纯、一根筋的孩子。

  She is a naive and stubborn child.

• 大国切不能“一根筋”，这个笔直的道理，拿到今天的舆论场反而看上去像是歪的。

  A big country cannot be single-minded. This straight-forward argument is considered skewed by public opinion.