Phenylketonuria is a common amino acid metabolic disorder. It is primarily due to the deficiency of phenylalanine hydroxylase, which prevents the conversion of phenylalanine to tyrosine, leading to a series of clinical symptoms. These can manifest as intellectual disability, neuropsychiatric symptoms, eczema, skin scratching, pigment loss, and a musty odor. The skin whiteness mentioned is not merely facial pallor; it affects the entire skin because the benefits of tyrosine are reduced, leading to decreased melanin synthesis. Consequently, the hair of affected children can also be lighter and tend to be brown.