What is your guess? A four-day-old boy with an abnormal metabolic screen.

A four-day-old boy with a positive result in newborn-screening testing was referred to the hospital for further evaluation. He was the product of a 39-week gestation and appeared well at birth. His laboratory results included the following plasma findings: ammonia, 109 g/dL (reference interval, 20 – 65 g/dL); citrulline, 169 mol/L (reference interval, 2–50 mol/L); and leucine, 278 mol/L (reference interval, 32–153 mol/L). Also found were increases in homocysteine and -aminobutyric acid. The amino acid spectrum is shown below (Fig. 1).