Urea Cycle

A 3-day-old male neonate born to non-consanguineous parents presents with poor feeding, lethargy, and vomiting. On examination, he is hypotonic and has a weak cry. Laboratory investigations reveal ammonia level of 380 µmol/L (normal <50), normal blood glucose, normal liver function tests, and normal organic acids on urine chromatography. Plasma amino acid analysis shows elevated glutamine and alanine. A diagnosis of urea cycle disorder is suspected. Which enzyme deficiency is most likely responsible, given the clinical presentation and biochemical pattern?

A 3-year-old boy from North India presents with recurrent vomiting, developmental delay, and seizures. Serum ammonia is markedly elevated at 180 µmol/L (normal <50). Urine orotic acid is elevated. Genetic testing confirms OTC deficiency. Regarding the biochemistry of this disorder, all of the following are true EXCEPT:

A 3-day-old male neonate born to non-consanguineous parents presents with poor feeding, lethargy, and vomiting. Serum ammonia is 280 µmol/L (normal <50), and urine orotic acid is markedly elevated. Genetic testing confirms ornithine transcarbamylase (OTC) deficiency. What is the most appropriate immediate next step in management?

A 5-day-old male neonate born to a healthy mother presents with lethargy, poor feeding, and a single seizure. Serum ammonia is 180 µmol/L (normal <50), blood urea nitrogen is 8 mg/dL (low), and urine orotic acid is markedly elevated. The enzyme marked **B** in the urea cycle diagram is deficient. Which of the following best explains the elevated urinary orotic acid in this patient?

A 6-month-old male infant presents with lethargy, poor feeding, and vomiting. Serum ammonia is markedly elevated at 180 µmol/L (normal <50). Plasma amino acid analysis shows elevated glutamine and alanine. Genetic testing reveals a homozygous loss-of-function mutation in NAG synthase. The structure marked **A** in the urea cycle diagram is the rate-limiting enzyme of ammonia detoxification. Which of the following best explains why NAG synthase deficiency presents clinically identical to CPS-1 deficiency despite normal CPS-1 protein expression?