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Inborn Error of Metabolism 1

Inborn error of metabolism
biochemistry

Question

Vignette: A 5-year-old boy is brought to the pediatrician by his parents due to recurrent episodes of vomiting and lethargy. His parents note that these episodes often occur after prolonged periods without eating. His growth and development have been normal. Laboratory studies show a low blood glucose level. Urine organic acid screening reveals a large amount of dicarboxylic acids and a decreased amount of ketone bodies. Which of the following enzymes is most likely deficient in this patient?

Choices

A. Carnitine acyltransferase I

B. Acetyl-CoA carboxylase

C. Pyruvate carboxylase

D. Pyruvate dehydrogenase

E. Beta-ketothiolase

Answer

A. Carnitine acyltransferase I

Explanation

This patient's recurrent episodes of vomiting, lethargy, and hypoglycemia following periods of fasting, as well as the presence of dicarboxylic aciduria and hypoketonemia, suggests a defect in fatty acid oxidation. During periods of fasting, fatty acids are the primary energy source for the body. They are transported into the mitochondria for oxidation via the carnitine shuttle, which consists of two enzymes: carnitine palmitoyltransferase I (CPT I) and II. CPT I, located on the outer mitochondrial membrane, converts fatty acyl-CoA to fatty acylcarnitine, which can be transported across the membrane. A deficiency in CPT I would inhibit this process, leading to decreased beta-oxidation, hypoketotic hypoglycemia, and accumulation of dicarboxylic acids in the urine due to increased omega-oxidation in the peroxisomes. The other enzymes listed do not directly involve in fatty acid oxidation.

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