Disease phenylketonuria (PKU)?

A woman with the rare recessive disease phenylketonuria (PKU), who had been treated with a diet having low levels of the amino acid phenylalanine, was told that it was unlikely her children would inherit PKU because her husband did not have it. However, her first child had PKU. Please list all the possible explanations (at least 3). What is the most likely one? Assuming this explanation is true, what would be the probability of her second child having PKU?

PKU is actually caused by a mutation of a gene called phenylalanine hydroxylase (PAH). Every person has two of these genes. In order for PKU to occur in a person, both copies of this gene must be mutated. In the example you gave, the mother already has PKU which means that both of her PAH genes are of the mutant variety. In order for any of her children to inherit this disorder the father must be a carrier. What this means is that he has one normal PAH gene and a mutant one. Since both genes must be abnormal, the father does not exhibit the disorder, but he is still a carrier for it. The offspring inherits one of the PAH genes from the mother and one from the father. Since both of the mother's PAH genes are abnormal we know that the child will get at least one mutant gene. Since the father has one normal, and one abnormal PAH gene there is a 50% chance that the child will inherit the abnormal gene from him also. If this occurs then the child will have PKU. If the child gets the normal gene, it will not develop PKU. In this situation for every child they have there would be a 50% chance of the child getting PKU and a 50% chance that they will only be a carrier for this. The fact that the woman in your example was treated with a diet low in phenylalanine makes no difference on the odds of the child getting the disease. A diet only helps treat the symptoms of PKU; it does not cure the disease.

You asked for 3 explanations for the cause of her child getting the disease, but this is really the only viable explanation unless the father of the child was actually another man, or the gene that was inherited from the father was normal, but somehow became mutated during replication, but that doesn't seem likely at all.

There are ample sources available on this topic. The one listed below is presented in an animated format that is really easy for people without a strong science background to understand.

Her first child is not the fathers.
Rare for one family to have two children with this.
I dont know why she was treated with the diet in the first place its the child who would need the diet.

Her husband is a carrier.As far as I'm aware,(my brother has PKU,and 2 of my cousins from my mothers side of the family),the chances of a child inheriting this disease are 1 in 4 but both parents have to be carriers(as in my family's case)or have PKU themselves.The first child in my cousins family didn't have this disease(neither did my aunt and uncle),the second and third children did!