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Urea Cycle Enzymes 1

Urea cycle enzymes
biochemistry

Question

Vignette: A 9-year-old boy is brought to the emergency department by his parents due to severe abdominal pain and vomiting. His parents report that he has been having repeated episodes of abdominal pain, vomiting, and lethargy since infancy. The boy’s growth and development have been normal. On examination, his blood pressure is 110/70 mmHg, pulse rate is 86/min, and respiratory rate is 18/min. Physical examination is unremarkable except for mild hepatomegaly. Laboratory tests reveal elevated blood ammonia levels. Urine organic acid analysis shows increased orotic acid. Which of the following enzymes is most likely deficient in this patient?

Choices

A) Arginase

B) Argininosuccinate synthetase

C) Carbamoyl phosphate synthetase I

D) Ornithine transcarbamylase

E) UMP synthase

Answer

D) Ornithine transcarbamylase

Explanation

The patient’s presentation of recurrent episodes of vomiting, lethargy, and elevated blood ammonia levels suggest a urea cycle disorder. The urea cycle is responsible for converting toxic ammonia into urea for excretion. Among the urea cycle disorders, the most common is Ornithine transcarbamylase (OTC) deficiency. This enzyme catalyzes the reaction between carbamoyl phosphate (derived from ammonia) and ornithine to form citrulline in the mitochondria. Deficiency of OTC leads to an accumulation of carbamoyl phosphate, which is then shunted into the pyrimidine synthesis pathway, resulting in excess orotic acid (as seen in this patient). This differentiates it from other causes of hyperammonemia, such as N-acetylglutamate synthase deficiency or carbamoyl phosphate synthetase I deficiency, which do not result in excess orotic acid. Males are typically more affected due to the X-linked pattern of inheritance.

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