noun
patellar reflex; knee-jerk reflex
Medical term for the reflex contraction of the quadriceps when the patellar tendon is tapped; also used in figurative contexts for an automatic, unthinking response.
医師は膝蓋反射を調べるために、膝の下を軽く叩いた。
The doctor lightly tapped below the knee to test the patellar reflex.
彼
His reaction was as quick as a knee-jerk reflex.