What enzyme deficiency is likely indicated by the symptoms of a 20-day-old male infant with vomiting, fussiness, and a musty odor to skin and urine?

The symptoms described in the question—vomiting, fussiness, and a distinctive musty odor to the skin and urine—are indicative of phenylketonuria (PKU), which results from a deficiency in the enzyme phenylalanine hydroxylase. This enzyme is crucial for the conversion of the amino acid phenylalanine into tyrosine. In individuals with PKU, the accumulation of phenylalanine leads to toxic effects on the brain and can cause developmental delays and intellectual disability if not managed through dietary restrictions.

The musty odor is particularly characteristic of PKU and arises due to the buildup of phenylalanine and its metabolites, which are excreted in the urine. Early detection and intervention, usually through newborn screening, can significantly improve outcomes for affected infants.

The other enzymes listed do not correlate with the specific symptoms presented. Tyrosinase is involved in melanin synthesis and doesn't relate to these symptoms. Urokinase is related to the breakdown of fibrin in blood clots and has no connection to the metabolic issues presented here. Arginase is involved in the urea cycle, and its deficiency leads to different clinical presentations, such as hyperammonemia, which does not typically feature the