He clenched his fists so tightly that his knuckles turned white.

knuckles 是 knuckle 的复数形式，knuckle 指手指关节，特别是手指与手掌连接处的关节。当握紧拳头时，这些关节会因压力而显得突出，有时会因血液流通不畅而发白。