Harvey Mann-Walker died at the age of 26 after suffering a "catastrophic and unsurvivable" brain injury following collapsed lungs and a cardiac arrest.
But an inquest into his death heard that just a week before suffering the injury, Covid restrictions cost him the opportunity to have a chest x-ray.
Mr Mann-Walker attended the Norfolk and Norwich University Hospital a year ago today (July 13) after calling 999 with chest pains and difficulties breathing.
The father-of-one was told to attend A&E by a paramedic and went accompanied by his mother Sarah Walker for support.
Miss Walker told the court that her son lived with ADHD and severe anxiety which meant he was unwilling to attend hospital unaccompanied.
So when he was told at the doors to A&E that his mother could not accompany him, he "panicked", turned away and left the hospital without being treated.
A week later, on July 20, his lungs collapsed, causing him to go into cardiac and respiratory arrest.
The court heard that the collapsed lung had stopped him breathing, resulting in his brain being starved of oxygen, from which he died.
It happened in his home in Magdalen Close, in Norwich, where he lived in a third-floor flat.
The incident was attended by paramedics from the East Anglian Air Ambulance, but he was taken by road to the same hospital that he had been to the week before.
Miss Walker believes that had she not been turned away from the hospital, signs of what was about to happen may have been found - and treated accordingly.
She said: "Anybody could see he was in agony but we were told that rules are rules.
"I firmly believe if we had not been turned away he would have been admitted straight away and put in a comfortable environment.
"This was preventable but now I am left with only memories of my 26-year-old son and his daughter Amelia, who is only two-and-a-half, has been left to grow up without her father, which feels immensely unfair.
"Had he been admitted a week earlier, we could have prevented this tragedy."
Miss Walker did, however, praise the care he received once he was admitted.
She said: "The care he received in critical care was second to none - the doctors who looked after him were professional, sensitive and caring.
"If only he could have survived long enough to thank them for that."
In a statement provided to the court, a legal representative for the hospital trust said that they had no record of Mr Mann-Walker's visit to the hospital on July 13, so could not comment on what staff had told him.
However, they confirmed the guidance the emergency department was working under at the time.
The representative confirmed that while patients were encouraged to attend A&E alone, they were allowed to be accompanied if they had extenuating circumstances or specific needs.
They added staff were encouraged to judge each case on its own merits.
Assistant coroner Christopher Leach, however, said he was satisfied that the missed opportunity for a chest x-ray did not contribute to Mr Mann-Walker's death.
Giving a narrative conclusion, Mr Leach said: "On July 20, Harvey Mann-Walker had an unexpected condition in which air gathered around both of his lungs.
"This resulted in cardiac and respiratory arrest which in turn starved his brain of oxygen resulting in a brain injury causing his death.
"This was a tragically sad case."