Aaron John McLeod, 40, has denied murdering his former partner's two-year-old son, who died after suffering catastrophic brain injuries and multiple fractures at his NSW mid-north coast home in May 2022.
Neither the boy nor his mother can be identified for legal reasons.
Specialist pediatrician Mary Passmore examined the child at John Hunter Children's Hospital in Newcastle, while he was intubated with a breathing tube the day before he died.
She described his injuries to the jurors presiding over McLeod's NSW Supreme Court trial on Wednesday.
The toddler presented to hospital with a fractured pubic bone, brain haemorrhage, a torn neck artery, broken blood vessels in his eyes, fluid in his lungs and a swollen brain, Dr Passmore said.
"This child had a catastrophic brain injury, is that right?" crown prosecutor Ben Allison asked.
"That is correct," Dr Passmore replied.
Before examining the two-year-old, the pediatrician sat down with the boy's mother to get his history.
She was told the child had fallen down four steps at the back deck of his home the day before he was sent to hospital.
At 3am that night, his mother woke to find her son on all fours out of his bed, banging his head on the carpet.
She told the pediatrician that she settled the boy and put him back to sleep.
The next morning, he couldn't be roused, the jury heard.
"He wouldn't wake up and appeared lifeless, he turned blue," Dr Passmore's notes said.
As his mother commenced CPR while paramedics were on their way, blood started to come out of his nose and mouth, the court was told.
McLeod has pleaded not guilty to murder, denying allegations he physically manhandled the boy after dating the child's mother for two months.
The jury was previously told all of the toddler's injuries did not stem from an accidental fall down the stairs or head-banging.
Dr Passmore seemed to confirm this on Wednesday.
She was frank when asked if the boy could have eaten dinner and played with his brother with such a significant injury.
"I would be surprised," she said in response to Mr Allison's question.
"I would expect him to be in pain and discomfort."
The two-year-old also did not exhibit any signs of vomiting or drowsiness expected from such serious head injuries, the doctor said.
"Taking all of that history ... does that describe what you would expect to see in the functioning of a child with this sort of brain injury at all?" Mr Allison asked.
"No, it's not what I would expect," Dr Passmore said.
In 12 years of assessing injured children, she said she could not recall seeing anyone who sustained life-threatening injuries by falling down stairs or injuring their brain by banging their head.
The trial before Justice Phillip Boulten continues.
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