All Sample Questions

Metabolic Disorder 1

Question

Vignette: A 5-year-old boy is brought to the pediatrician by his parents due to frequent episodes of vomiting, abnormal behavior, and lethargy, especially after high-protein meals. His growth and development have been normal until now. His neurological exam shows mild ataxia and nystagmus. Laboratory tests reveal increased levels of glutamine, alanine, and aspartate in blood and urine. Which of the following enzymes is most likely deficient in this child?

Choices

A) Glutamate dehydrogenase

B) Alanine aminotransferase

C) Carbamoyl phosphate synthetase I

D) Ornithine transcarbamylase

E) Aspartate aminotransferase

Answer

D) Ornithine transcarbamylase

Explanation

The child's symptoms and the presence of increased glutamine, alanine, and aspartate in blood and urine suggest a disorder of the urea cycle, which is responsible for the conversion of highly toxic ammonia into urea. Among the enzymes involved in the urea cycle, Ornithine transcarbamylase (OTC) is the most commonly deficient and is associated with an X-linked recessive inheritance pattern. OTC deficiency leads to hyperammonemia, characterized by vomiting, abnormal behavior, lethargy, ataxia, and nystagmus due to the accumulation of ammonia in the brain. The increased levels of glutamine, alanine, and aspartate are due to the increased use of these amino acids as alternative routes to detoxify ammonia when the urea cycle is impaired.