Inborn Errors of Metabolism and Newborn Screening Quiz

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Which inheritance pattern is common among enzyme-deficiency inborn errors of metabolism?

A Autosomal recessive inheritance ✓
B Maternal mitochondrial inheritance
C Autosomal dominant inheritance
D X-linked dominant inheritance
E X-linked recessive inheritance

The answer is A: Autosomal recessive inheritance

Explanation

Why A is correct

See Inheritance and Variable Severity. Option A is correct. An inborn error of metabolism is usually an inherited defect in an enzyme, a transporter or a cofactor-handling step. Many enzyme deficiencies are autosomal recessive: the gene is on one of the non-sex chromosomes (an autosome), and a person is affected only when they inherit two altered copies, one from each parent. Each parent, who carries just one altered copy, usually keeps enough enzyme activity (a reserve) to stay well, which is why carriers are often healthy and why the condition can appear unexpectedly in a family. Phenylketonuria (PKU) and McArdle disease are examples. The pattern is common, but it is not universal, and important exceptions exist, including X-linked disorders (such as Lesch-Nyhan disease) and mitochondrial disorders, so inheritance should be checked for the specific disease. Memory aid: recessive means two copies are needed, and carriers are protected by their spare copy.

Common pattern: autosomal recessive, so two altered copies are needed, one from each parent.
Carriers: they have one altered copy and usually keep enough enzyme activity to stay well.
Exceptions: X-linked and mitochondrial disorders exist, so inheritance should be checked for the specific disease.

Why the other options are incorrect

B. Maternal mitochondrial inheritance: Maternal mitochondrial inheritance does occur for some metabolic disorders and is one of the recognised exceptions. It is not the common pattern among enzyme-deficiency inborn errors, so it is not the best answer.
C. Autosomal dominant inheritance: Autosomal dominant inheritance can occur in metabolic disorders, but it is not the common pattern for enzyme-deficiency inborn errors. With autosomal recessive inheritance, carriers with one altered copy usually retain enough enzyme activity to stay well.
D. X-linked dominant inheritance: X-linked dominant inheritance is uncommon among enzyme-deficiency inborn errors of metabolism, and it does not explain the usual autosomal recessive pattern seen in many enzyme deficiencies.
E. X-linked recessive inheritance: X-linked recessive inheritance occurs in some inborn errors, such as Lesch-Nyhan disease, but it is an exception rather than the common pattern. Most enzyme-deficiency inborn errors require two altered alleles on autosomes.

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