During the prosecution’s opening speech, it emerged that Ms Bednarczyk was pronounced dead at the scene after her body was pulled from the smoke-filled living room of the property into a hallway by firefighters.
A Home Office pathologist subsequently examined Ms Bednarczyk’s body, finding wounds to her face, neck and back, the trial was told.
Mr Skinner said of the defendant: “She used more than one knife. The pathologist examined Marta’s body and found there were at least 143 sharp force injuries – 65 were in her head and her neck.
“Seven were in the front of her torso, 33 were in her back, 10 were in her arms and 18 on her hands and wrists.”
One of the injuries entered the victim’s brain while two others entered her lungs.
Mr Skinner said of the wound which entered the skull: “The pathologist said that the force needed to do that with the knife was severe.”
After the jurors were given initial legal directions by the trial judge, Mr Skinner told them the girl was claiming diminished responsibility, which would make her guilty of manslaughter rather than murder.
He told the panel: “In truth this issue of diminished responsibility is likely to be the main focus of your attention in this trial because we, the prosecution, do not accept that she has the benefit of the defence of diminished responsibility.
“We say that we will make you sure that this is a case of murder because she intended to do really serious harm.
“We say it is murder because she planned the killing, and we say it is murder because she lied about what she did.
“And we say it is murder because there are genuine specialists in this field of psychiatry and psychology that say her actions were not caused by poor mental health.”