She's very single-minded (literally: only one tendon in her brain).
Tips
usage
一根筋 (yī gēn jīn) is a colloquial expression meaning stubborn or single-minded. 伤筋动骨 (shāngjīn dònggǔ) means a serious injury involving tendons and bones.
memory
The bamboo radical 竹 on top (because tendons are fibrous like bamboo) + 肋 (rib-like structure) + 力 (strength) — tendons give your muscles strength.
