When teaching the mother of a child diagnosed with phenylketonuria (pku) about its transmission, the nurse should use knowledge of which factor as the basis for the discussion?

The answer to this question would be: Autosomal recessive gene.

Phenylketonuria is an inborn error of metabolism that caused by defect gene that inherited by autosomal recessive. That means the disease is not transmitted by contact as it wasn't caused by pathogens. 
It needs two recessive genes to be expressed so the father and mother must have at least one defective gene.