The mother of an epileptic teenager who drowned in a bath at an NHS care unit said it "never crossed her mind" staff would not supervise her son.
Connor Sparrowhawk was found dead after being left on his own and suffering a seizure at Slade House in Oxford.
His mother Sara Ryan has given evidence to a Medical Practitioners Tribunal Service misconduct hearing.
It is considering the case of Dr Valerie Murphy, who was employed by Southern Health NHS Foundation Trust.
She denies misconduct, but has accepted she did not consider the implications of allowing him to use a bath alone, although he was checked on by staff every 15 minutes.
The 18-year-old's death on 4 July 2013 led to the discovery the trust had not properly investigated 272 unexplained deaths in its care.
Dr Murphy has admitted 30 separate failures including not obtaining consent for treatment from Connor or his parents and keeping proper medical notes.
'Situation was terrifying'
Ms Ryan, an Oxford University academic, fought back tears as she took the oath before giving evidence at the hearing in Manchester.
She said: "We just assumed he was being supervised in the bath. It was not something that came to my mind. It was almost one-to-one support on the unit.
"It had a whole team of specialist staff and there were five patients. It just never crossed my mind."
Ms Ryan added: "Connor had gone into that unit a family member and schoolboy. He was sectioned on the first night.
"I was told he was an adult and could not just turn up to see him. The whole situation was terrifying really."
Dr Murphy could be struck off if the hearing, which is due to run until 18 August, decides her fitness to practise is impaired.