​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 μg/dL (reference interval, 20–65 μg/dL); citrulline, 169 μmol/L (reference interval, 2–50 μmol/L); and leucine, 278 μmol/L (reference interval, 32–153 μmol/L). Also found were increases in homocysteine and γ-aminobutyric acid. The amino acid spectrum is shown below (Fig. 1). 

Fig. 1. Amino acid spectrum for the patient.

The patient's plasma sample was analyzed on a Hitachi L-8800 amino acid analyzer after removing proteins by treatment with 40 g/L sulfosalicylic acid/internal standard solution, centrifugation, and filtration. The amino acids were separated by high-resolution ion-exchange chromatography and then subjected to postcolumn ninhydrin derivatization; detection was by spectrophotometry at 570 nm and 440 nm. The areas under the curves were calculated by using the manufacturer's software for final quantification. Hcys, homocysteine; γ-ABA, γ-aminobutyric acid; AEC, S-2-aminoethyl-l-cysteine; 3-M Histidine, 3-methylhistidine. 


  1. What class of genetic disorders did this patient most likely have?
  2. What enzyme deficiency was suggested by the patient's amino acid profile, and how did you reach the conclusion?
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