A 1-week-old boy is brought in by his mother to the emergency department for evaluation of a new onset seizure at home which occurred one hour ago. The mother states he has vomited several times today and has not been feeding well since yesterday. The infant was born healthy at term without complications. The mother had routine prenatal care throughout her pregnancy. On physical examination, the infant has a weak cry and appears to have low muscle tone. His diaper is notable for a sweet scent. Serum and urine amino acid tests are ordered which confirm the diagnosis.
Which of the following amino acids should not be given to this infant?
- A) Arginine
- B) Leucine
- C) Phenylalanine
- D) Tryptophan
- E) Tyrosine
This infant has maple syrup urine disease. It is caused by a build-up of branched-chain amino acids (leucine, isoleucine, valine) due to decreased branched-chained α-ketoacid dehydrogenase. Presentation includes poor feeding, vomiting, seizures, and most notably the aroma of maple syrup in the urine. Treatment is thiamine supplementation and restriction of leucine, isoleucine, and valine in the diet.
Answer choice: A: Arginine, is incorrect. Arginine is avoided in those with arginase deficiency. This disorder causes the build-up of arginine and ammonia in the blood. Presentation includes poor feeding, poor growth, seizures, and spasticity. The sweet aroma will not be found in those with arginase deficiency. Treatment is with a low protein diet, arginine restriction, and supplementation with essential amino acids.
Answer choice: C: Phenylalanine, is incorrect. Phenylalanine is avoided in those with phenylketonuria (PKU), due to the inability to convert phenylalanine into tyrosine. Presentation includes intellectual disability, microcephaly, seizures, hypopigmented skin, and a musty odor. Treatment is by restricting phenylalanine in the diet and supplementing tyrosine.
Answer choice: D: Tryptophan, is incorrect. Tryptophan is not a branched-chain amino acid and may be consumed by those with maple syrup urine disease.
Answer choice: E: Tyrosine, is incorrect. Tyrosine is not a branched-chain amino acid and may be consumed in those with maple syrup urine disease.
Key Learning Point
Maple syrup urine disease is caused by a build-up of the branched-chain amino acids leucine, isoleucine, and valine due to decreased branched-chained α-ketoacid dehydrogenase. Treatment is thiamine supplementation and restriction of leucine, isoleucine, and valine in the diet.
Dr. Ted O'Connell