Young Dro's GF told cops the violence erupted in the bedroom, when he punched her so hard it left a bruise. She says a while later she was eating dessert on the sofa when Young Dro smashed the plate of banana pudding in her face. Then she says he grabbed a kitchen knife and went to town on the living room furniture.

As we reported ... when cops showed up, they arrested Young Dro, despite the fact his GF did not want to press charges, presumably because they saw she was injured.

For his part, Young Dro told cops he didn't strike her during their altercation ... and claimed the bruise was actually a birthmark.

Dro's girlfriend told police he has a drug problem and needs help. She said he takes pills -- Xanax, Ecstasy, Albuterol, Molly, Percocet -- and lashes out when he comes down from his high.

Young Dro is charged with misdemeanor battery/family violence.