Dean Fitzpatrick died from a single stab wound to his stomach which was inflicted with moderate or considerable force, the Central Criminal Court has heard.
But Deputy State Pathologist Dr Michael Curtis said his examination could not tell if the wound had been inflicted with a deliberate thrust of the knife or if Mr Fitzpatrick had "run on" to a knife being held by someone else.
David Mahon, 46, who is the partner of Mr Fitzpatrick's mother Audrey, denies murdering the 23-year-old.
He told gardaí his stepson's death was an accident.
Dr Curtis said the wound was around 14.5cm deep and left a groove on Mr Fitzpatrick's spine.
He said Mr Fitzpatrick essentially bled to death due to the damage inflicted by the stab wound.
He said that although Mr Fitzpatrick had emergency treatment he succumbed to what was essentially a non survivable injury.
Dr Curtis said it would have required moderate or considerable force to inflict the injury on Mr Fitzpatrick.
He said his examination could not determine whether the injury had been inflicted with a deliberate thrust of a knife or whether Mr Fitzpatrick could have "run on" to a knife being held by Mr Mahon.
He said the nature of the wound to Mr Fitzpatrick meant either scenario was possible.
The wound was horizontal and tracked just slightly upwards.
The court heard a number of tranquilising drugs, known as "benzos", were found in Mr Fitzpatrick's system, some in trace amounts.
He also had scars on his forearm, indicating an episode of self-harm in the distant past. He had a black eye which was healing.
Under cross-examination by lawyers for Mr Mahon, Dr Curtis agreed there was no evidence of a second stab wound or that there was any twisting of the knife.
He said the depth of the wound could have been as little as 12.5cm and if a knife longer than that was used, it would mean the knife was not pushed in all the way.
He agreed this injury could cause massive internal bleeding with very little significant external bleeding.
Dr Curtis agreed with Defence Counsel Sean Guerin that it was "possible" that the injury was caused by Mr Fitzpatrick running onto the knife.
But he said it was also possibly the result of a deliberate thrust.
He agreed with Mr Guerin that the suggestion made by the prosecution that the jury would have difficulty squaring Mr Mahon's account of what happened with the pathological evidence was not a suggestion he ever made.
He agreed he was "absolutely" not giving that evidence.
Dr Curtis agreed that he was a hunter and knew what a "gutting" was.
He agreed with Mr Guerin that what Mr Fitzpatrick suffered was not a gutting.
Prosecuting counsel, Remy Farrell had suggested in his opening speech that Mr Fitzpatrick had been "gutted".
Trial hears from victim's father
Mr Fitzpatrick's father earlier told the Central Criminal Court that doctors tried to save his son twice but could not bring him back.
Christopher Fitzpatrick told the court that he had identified the body of his son on 26 May 2013.
He said his son had been living with him in the week before his death and he got a call from gardaí in the early hours of 26 May.
He went to Beaumont Hospital with his sister and brother-in-law, where the surgeon who operated on his son, told him they had tried to bring Dean back twice but were unable to save him.
The court heard Dean was pronounced dead at 12.50am.
Mr Fitzpatrick said there had been stuff going on with his son in the last week of his life in relation to being able to see his child and because his sister was missing.
Dean Fitzpatrick's sister Amy went missing in Spain in 2008.
Garda tells court accused came forward
This afternoon, the court heard Mr Mahon told gardaí investigating his stepson's death that he did it.
But Mr Mahon claimed the 23-year-old walked onto the knife he was holding.
Garda Patrick Brodigan told the court he was in Coolock Garda Station on the morning of 26 May 2013 when he got a call from a man who said he was David Mahon.
He said Mr Mahon talked about the death of Mr Fitzpatrick in the early hours of the morning, and he said he did it.
Mr Mahon said he did not know Mr Fitzpatrick was dead until he heard it on the news.
Mr Mahon came into the garda station a short time later, accompanied by a friend, John McCormack.
Garda Brodigan said Mr Mahon was talking constantly, he was extremely upset and crying intermittently.
The court heard Mr Mahon told gardaí that he and Mr Fitzpatrick had been having rows for years.
He said Mr Fitzpatrick had pulled knives and guns on him in the past. He said the previous night Mr Fitzpatrick had pulled a knife on him and he had taken it off him.
He said he showed it to Mr Fitzpatrick and asked him what he was doing, pulling a knife on his father.
Mr Mahon said Mr Fitzpatrick walked into the knife, putting it up to him and then ran off.
He told gardaí a "fella" was dead, it was his fault but he said Mr Fitzpatrick genuinely walked into the knife.
He told the gardaí that he said to Mr Fitzpatrick that he was going to stab him because he said that was the language Mr Fitzpatrick used.
He said Mr Fitzpatrick was a little bastard but did not deserve this. He said he did not stab Mr Fitzpatrick: he walked into the knife, but he did not know if that made any difference.
Mr Mahon said he did say he was going to stab Mr Fitzpatrick but he did not mean it.
He said he had been drinking on the previous day but said he was not drunk. He said he used to drink heavily for about a year or so.
He referred to a book on Amy Fitzpatrick that he had previously given to one of the garda sergeants.
He said he rang Mr Fitzpatrick to get him to have a chat with him after his partner Audrey, Mr Fitzpatrick’s mother, told him he had to sort him out.
Asked why he did not call for medical assistance, Mr Mahon said he did not think Mr Fitzpatrick was that badly hurt as he did not see blood and he ran off.
He asked gardaí how he was going to face his partner. He said he did not mean to kill Mr Fitzpatrick.
Mr Mahon said he threw the knife out the window of his friend Karl O'Toole's car and he did not remember where.
He was asked if he wanted to consult with a solicitor and he declined.