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Ornithine transcarbamylase deficiency

Summary of Ornithine transcarbamylase deficiency
Ornithine transcarbamylase deficiency is the most common urea cycle disorder in humans. It is inherited in an X-linked manner. The defective enzyme, ornithine transcarbamylase, is the final enzyme in the proximal portion of the urea cycle, responsible for converting carbamoyl phosphate and ornithine into citrullaine. In severe cases, rapidly increasing ammonia concentrations cause ataxia, lethargy and death without rapid intervention. This deficiency is diagnosed through clinical findings and biochemical testing, confirmed with molecular genetics techniques. Treatment aims to avoid precipitating increased ammonia levels. The most common treatment combines a low protein diet with nitrogen scavenging agents. Liver transplant is considered curative for this disease.

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Biochemistry and nutrition

Biochemistry

Biochemistry and metabolism
Metabolic disorders

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Ornithine transcarbamylase deficiency

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Ornithine transcarbamylase deficiency

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USMLE® Step 1 style questions USMLE

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A 2-week-old male infant comes to the emergency department because of unusual irritability and lethargy. Patient is admitted to the pediatric intensive care unit and minutes later develops metabolic encephalopathy. This progressed to a coma, followed by death, before any laboratory tests are completed. Infant was born at home via vaginal delivery at 39 weeks’ of gestation. His mother says that the symptoms  started since the infant was 4-days-old, but since he only seemed “tired,” she decided not to seek for medical attention. Further testing during autopsy shows hyperammonemia, low citrulline and increased orotic acid. Which of the following enzymes is most likely deficient in this patient?