Rezwan Ali, 19, was knifed three times in total while in the kitchen of a house on Willows Road, in Walsall, Birmingham Crown Court heard.
But forensic pathologist Dr Alexander Kolar said it was one of the those strikes that delivered the fatal blow on January 14, 2018.
Mr Ali, a pizza delivery driver, suffered a wound "in excess of four inches" to his chest which pierced his heart.
The trial so far:
Jurors were previously told Mr Ali bled to death following the viscous assault which was sparked by a trivial argument between two groups of young men.
Dr Kolar said: "It is a singular stab wound to the chest that I give as the cause of death.
"The other stab wounds may have contributed to the bleeding but this was the cause of death.
"Essentially, this has gone through from the skin surface, through the muscles, through the rib cage and through the heart.
"This is the fatal injury. It has caused significant damage to the heart.
"In terms of mobility and actions, some people will collapse straight away while others will have a few minutes of meaningful activity."
Witnesses told jurors they found Mr Ali lying on the floor after he had been attacked.
Mr Ali also received a second stab wound to his chest which pierced his diaphragm.
He also received a stab wound to his arm, also in excess of four inches, that cut a "cutaneous nerve" which allows a person to flex their arm and elbow, said Dr Kolar.
Rajan Natt, 20, of Castle Street, West Bromwich, is standing on trial accused of Mr Ali's murder, which he denies.
The trial continues.