Venod Skantha (32) is accused of murdering 16-year-old Amber-Rose Rush, who was found dead in her Corstorphine home on February 3 last year.
The prosecution said at the start of the trial in the High Court at Dunedin that the defendant stabbed the victim to death about midnight the night before, following a heated online dispute.
Amber-Rose threatened to make allegations of sexual offending to both Skantha's hospital bosses and police, and posted screenshots of their exchange on Instagram.
The desire to protect his fragile career drove the man to murder, the Crown says.
Forensic pathologist Dr Kate White, who completed her autopsy on February 5, 2018, yesterday told the court Amber-Rose died "as a result of an incised wound to the left side of the neck".
The injury that caused her to bleed to death sliced through her earlobe and hit the base of her skull.
Dr White told the jury the 11cm-long wound hit her carotid artery and breached the windpipe.
Crown prosecutor Robin Bates cautioned the jurors before providing them with photo booklets depicting Amber-Rose's body. Family members chose not to attend that portion of the hearing.
He asked the pathologist how much force was likely to have been required to cause such damage.
"It's a very difficult question to answer and there's no way of precisely answering that," Dr White said.
She said the damage to bone suggested at least moderate force but that was also dependent on the sharpness of the knife used.
Severing the carotid artery, which takes a large volume of blood to the brain, meant the victim would have "bled out" without medical attention.
"How long would it have taken?" Mr Bates asked.
"Some minutes," Dr White said.
As well as the fatal injury, she also identified stab wounds to the back of the neck and two "superficial" horizontal cuts to the girl's throat.
None of those would have contributed materially to her death, the pathologist said.
In cross-examination, Jonathan Eaton QC asked whether the witness could determine whether the killer was right or left-handed.
It was "notoriously unreliable" to speculate on that, Dr White said.
She did not measure the depth of the wounds because it was an unreliable way of estimating the length of the blade used.
She had not examined any weapon which allegedly caused the injuries.
The jury was later shown CCTV stills that put a car matching Skantha's silver BMW on the route to the victim's home that night.
Two cameras on South Rd showed what the Crown says is Skantha travelling from Caversham, heading east, at 11.38pm - 13 minutes after his online spat with Amber-Rose.
It was a two-door vehicle with tinted windows, with the "distinctive appearance of being higher at the rear than front", police said.
Seven minutes later, the car is seen heading in the opposite direction.
The Crown case is that Skantha had picked up his teenage friend and they were on their way to Amber-Rose's Corstorphine home.
Extensive evidence from police telecommunications expert Justin Harris also broadly put the pair's cellphone in the general areas at times the prosecution say the murder was committed.
The trial, before Justice Gerald Nation and a jury of 10 men and two women, will restart on Monday with evidence from the key witness, the teenager who was allegedly with the defendant before and after the incident.
