apple of discord

Something that causes unhappiness or trouble. That stupid prom dress has been their apple of discord—they're been fighting over who gets to wear it for days now! Why aren't you talking to Mary Beth? What is the apple of discord here?

Common Names:

Name	Pronounced	Usage
Marquis	mahr-KEE	African American
Fatimah	fah-TIM-ah (Arabic)	Arabic, Malay, Indonesian
Sargon	SAHR-gawn (English)	History
Fuad	-	Arabic
Hannah	HAN-ə (English), HAH-nah (German)	English, Hebrew, French, German, Dutch, Swedish, Biblical
Marni	-	English