Biology disease question...?

Question

A couple has a female child with phenylketonuria, and 3 unaffected children. Neither parent nor any of the 4 biological grandparents of the affected child has had this disease. The most likely genetic explanation is that phenylketonuria is inherited as a(n) ________ disease.

1. autosomal dominant
2. autosomal recessive
3. sex-linked recessive
4. sex-linked dominant
5. cannont make a reasonable guess from this information

Answer 1

2. autosomal recessive

Can't be dominant or it would have shown up in some of the previous generations.
Isn't sex-linked because the father would have had to had PKU.

Answer 2

2 autosomal recessive