A pathologist today said the injuries suffered by a tragic toddler were "highly suspicious of fatal child abuse".
Jonathan Simpson is accused of murdering Jacob Marshall while the little boy's mum was at the hairdressers.
He is alleged to have inflicted multiple injuries including a "catastrophic brain injury" on Friday, July 12, 2019.
Prosecutors say Emma Marshall's then boyfriend has given "several different explanations" for Jacob's death - first claiming he fell off a couch, later telling police "I dropped a baby down the stairs", and finally suggesting in a prepared statement to police that Jacob fell down the stairs.
Liverpool Crown Court has heard Simpson asked a neighbour in Belsford Way, Speke for help at around 3.45pm.
The neighbour said he found Jacob unconscious in the kitchen, with a large bruise to the right side of his head, and had to prompt Simpson to call an ambulance, before the alleged killer went for a cigarette.
Paramedics attended at 4.10pm and were concerned by unexplained bruises to Jacob's body, including to his head, groin, arms, shins and feet.
Doctors at Alder Hey Children's Hospital found "suspicious" injuries including bruising to his penis and his left ear.
A CT scan revealed a subdural haemorrhage - blood compressing his brain - and extensive bleeds to his eyes.
Nothing could be done to save Jacob, who was pronounced dead late on July 13.
Home Office pathologist Dr Jonathan Medcalf carried out two post-mortem examinations on Jacob.
Dr Medcalf today said "very severe" bleeding in the eyes, including to optic nerves, were "particularly significant".
He said these retinal haemorrhages were most likely caused by head trauma involving "high energy", typically associated with car crashes and "high level" falls.
Dr Medcalf said there was extensive research into falls down stairs by children of Jacob's age and it was "very rare" they resulted in death.
He added: "It's very rare also that more than one body region is injured."
The doctor said: "It's not impossible, of course young kids can die from falling down the stairs, it does happen, but it's a rare event. My view is with all the other findings, it doesn't fit."
Dr Medcalf said neuropathologist Dr Daniel Du Plessis found the head trauma would have required a substantial head impact and shaking alone was unlikely to account for the bleeding on the brain.
He said Dr Du Plessis believed it would have been "exceptional" if the brain injuries were explained by Jacob falling off a couch and hitting his head on a radiator.
Dr Medcalf said fatal head injuries caused by falling down stairs were "exceptionally rare" and multiple injuries involving various body parts "very unusual".
He said serious injuries when a child falls down stairs usually involve a child being dropped by an adult and therefore falling from a greater height.
Reading from his report, Dr Medcalf said there were six separate areas of bruising to the forehead.
He said two bruises, one either side of Jacob's forehead, were consistent with injuries his mum already knew of.
However, he said none of the other bruises had features indicating they were significantly older than July 12.
The doctor said a vertical mark in one forehead bruise seemed to fit with Jacob running into a door that morning - an explanation Simpson gave Miss Marshall when she returned after dropping his older brother off at school - but it was "unclear" how this caused his nose to bleed, which was also noted by the mum.
Dr Medcalf said the bruising to the left ear was "highly suspicious", even in the context of a fall down stairs.
He said: "The sheer number of injuries to this child are very worrying and some injuries of themselves are deeply suspicious of abuse."
The doctor said multiple small bruises to Jacob's genitals were "suspicious of pinching of the skin".
He said young boys occasionally do this to themselves, but according to his mum, Jacob wasn't known to do this and she hadn't seen this injury before.
Dr Medcalf was also worried by fingertip bruising to the jaw, consistent with gripping, and bruising to the pubic area, abdomen and feet.
He said: "Whilst on the face of it, a fall down the stairs would seem to reasonably account for multiple bruises to the body involving various anatomical regions, it should be noted it is unusual, or very unusual, for injuries to be so widespread after such falls."
Dr Medcalf also suggested it was "odd" there was no grazing or "carpet burns", which he would have expected from a fall down carpeted stairs.
He said changing the explanation of how injuries are caused was "a feature frequently seen in child abuse".
John Benson, QC, prosecuting, asked Dr Medcalf to outline nine areas of concern.
Dr Medcalf said: "In my opinion the overall features in this case are highly suspicious of fatal child abuse."
He said the nine points were "the carer's changing story; the distribution and number of injuries; the lack of abrasion; ear bruises; oral bruises; mandible [jaw] bruises; abdominal and pubic bruises, penile bruises; and florid retinal haemorrhages".
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The doctor said even taking into account the description of Jacob as "clumsy" by his mum, "I do not believe the account given in the carer's prepared statement adequately explains the findings".
He found the cause of death was a head injury.
Gordon Cole, QC, defending Simpson, said when Dr Metcalf read from his report, he had inserted the word "very" when saying it was "very unusual for injuries to be so widespread after such falls".
Asked why he inserted this word, Dr Medcalf replied: "Well I wrote this report over a year ago and I have had more time to consider the aspects. I've also given evidence in the Family Court about this case, a lot has happened since I wrote this report.
"My opinion is even stronger now - it's very unusual."
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The jury has heard Jacob previously attended A&E on April 20, 2019 with a head injury, when Miss Marshall told doctors he ran through an unlocked baby gate and she heard a noise and found him at the bottom of the stairs.
Dr Medcalf said: "We have a previous incident in this case, a fall down the same stairs with the same child and there is a dramatic difference in the findings following that witnessed fall, with a bump to the head with abrasion, and then abrasion to the back of one of the shoulders.
"So two injuries found, not numerous injuries to multiple parts of the body."
Dr Medcalf said giving different accounts of how injuries were caused was "a very significant feature of cases of child abuse", known as a "red flag".
He said Simpson's prepared statement was "vague and lacking in detail" and "inconsistent with the injuries to the child", which he thought was "critical in this case".
The doctor said a delay in seeking "appropriate care", which "in my view happened here", was also "very significant".
He said he believed a "reasonable adult" with an unconscious child they think has fallen down stairs should contact the emergency services.
Dr Medcalf said he didn't believe a fall down the stairs accounted for the injuries.
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Mr Cole said the witness had used the word "suspicious" on more than one occasion.
He said: "Do I understand this, while suspicious and very unusual, you can't rule out that that was a possible method of this little boy sustaining - and I don't mean all of the injuries - but sustaining the fatal injuries?"
Dr Medcalf said: "Young children do fall down the stairs, but the account given does not explain the injuries."
Mr Cole said he was referring specifically to the retinal haemorrhages and subdural haemorrhages.
Dr Medcalf said: "It's very hard to believe a fall down the stairs accounted for those, but like many things in medicine, it's not impossible."
Mr Cole later asked: "Can you actually rule out 100% a fall down the stairs?"
The doctor said: "I cannot rule it out as part of the fatal incident, a fall or a push or a throw down the stairs for example."
Simpson, of no fixed address, but from Winsford, Cheshire, denies murder and manslaughter.