No rule for this one
No ending or word group predicts the article of Arzt. Learn it as a pair and say it out loud: der Arzt.
der Arzt
It's der Arzt. Arzt is masculine, plural die Ärzte.
Der Arzt kommt gleich.
The doctor is coming soon.
No ending or word group predicts the article of Arzt. Learn it as a pair and say it out loud: der Arzt.