No rule for this one
No ending or word group predicts the article of Rücken. Learn it as a pair and say it out loud: der Rücken.
der Rücken
It's der Rücken. Rücken is masculine, plural die Rücken.
Ich habe Rückenschmerzen.
I have back pain.
No ending or word group predicts the article of Rücken. Learn it as a pair and say it out loud: der Rücken.