His question about her past touched a nerve and she became defensive.

touched 是动词 touch 的过去式，意思是"触碰、触及"，这里用作比喻义。 a 是不定冠词，表示"一个"或"某种"。 nerve 本义是"神经"，引申为"敏感点"或"痛处"，指容易引起情绪反应的话题或事情。 touch a nerve 是固定短语，字面意思是"碰到神经"，比喻"触及敏感话题"或"戳中痛处"。