The parents of a boy with a rare genetic condition have gone to the British High Court to fight the NHS's decision to refuse funding for a "life-changing" drug.
The seven-year-old, who a judge has said should be identified only as S, has severe autism and phenylketonuria (PKU), which inhibits his ability to digest protein.
His autism affects his ability to comply with the dietary restrictions which form the standard treatment for PKU, so his doctors sought funding for a drug called Kuvan.
S's counsel, Ian Wise QC, told Mrs Justice Andrews in London on Tuesday that funding had been repeatedly refused by NHS England with differing reasons given, leaving S at risk of the consequences of poorly-controlled PKU, including brain damage.
He said the decision to refuse funding should be quashed as it was "irrational, unreasonable and unlawful", as well as contrary to the 2004 Children Act and Article 8 of the European Convention on Human Rights, which covers private and family life.
S's father has told the court that his management had become more difficult over time because he would not take his supplements and was becoming more and more interested in normal foods since going to school and seeing what other children were eating.
The only method the family could use to maintain his phenylalanine levels was to seriously reduce his food intake.
"It is awful to make your child miss meals but the alternative, which is a risk of cognitive damage and all the other problems associated with too high phenylalanine levels, is worse," he said.
If prescribed Kuvan, S - who is of low weight and short for his age - would be able to get a proportion of vitamins and minerals from ordinary food.
His doctors say that every possible strategy had been considered, including admission to hospital, but the only potential solution is Kuvan.
S's father has branded the decision "extremely frustrating, unfair and inhumane".
"No family should have to fight with the NHS for over 18 months to ensure that their child is given proper treatment.
"It is clear to me they do not want to fund this life-changing drug."
At the start of the two-day contested hearing, the judge said it was important to clarify the court's role.
"If a rational conclusion was reached on the material then, however sympathetic one might be to this little boy's plight, the court cannot interfere.
"It is very important to explain the limits to which a court can intervene.
"I cannot substitute my view for that of these experienced clinicians.
"It is not for a judge to decide what is in the best interests of S.
"The question is whether this decision is susceptible to being interfered with on public law grounds."