A 1-year-old girl is brought to her pediatrician’s office with concerns about her
development. She had an uncomplicated birth outside the United States at
term. The mother reports that the baby is not achieving the normal milestones
for a baby of her age. She also reports an unusual odor to her urine and some
areas of hypopigmentation on her skin and hair. On exam, the girl is noted to
have some muscle hypotonia and microcephaly. The urine collected is found
to have a “mousy” odor.
What is the most likely diagnosis?
What is the biochemical basis of the hypopigmented skin and hair?