'He had a mass coming out of his pelvis and I panicked and wondered if he had a type of cancer.'
Checks revealed he had appalling constipation and his bowel had dilated.
'His bladder wall was so thickened,' she said.
'When I took the history in better detail this boy was gaming so much that his bladder and bowel were so deformed because he had stopped going to the loo.'
She said he had been so distracted by gaming he had ignored basic bodily urges.
Dr Begent, who was addressing the annual conference of the NSPCC, said gaming and social media were increasingly affecting the physical and mental health of youngsters.
The issue had 'crept up on us', she warned, with an exponential rise of cases in recent years.
She added: 'The sleep and obesity issues are the things we first started noticing – but these more profound physical health issues are just dawning on us now.'
Some children risk their hormones being affected because they are not sleeping enough.
In dramatic cases, Dr Begent believes young girls who are ignoring meals to be able to stay glued to their devices could be at risk of stunting their development.
She said: 'Hormonal pubertal development can be affected if they are low weight. And indeed obesity can affect pre-pubescent development as well, if you are very obese you can get polycystic ovaries – that might affect your periods.'
Worrying about beating another player on video games such as Fortnite is also damaging children's bodies far more than just a fleeting moment of stress, she warned.