Molecular biology and genetics: MELAS: practice question

A Biochemistry question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Biochemistry · Molecular biology and genetics: MELAS

Sample item

A 14-year-old has recurrent stroke-like episodes, lactic acidosis, seizures and short stature. Muscle biopsy shows ragged red fibres, and his mother has migraine and deafness. How is the disease inherited?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

C. Mitochondrial DNA mutation inherited maternally, with heteroplasmy

MELAS (mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes) is most often due to a point mutation in the mitochondrial tRNA-Leu gene. Mitochondrial DNA is passed only from the mother to all her children, and the proportion of mutant mitochondria varies between tissues (heteroplasmy), which explains variable severity. Ragged red fibres reflect proliferation of abnormal mitochondria.

Option by option

  • A. Autosomal dominant with full penetrance

    Autosomal dominant disease would pass from either parent, and affected fathers would transmit it.

  • B. Autosomal recessive with consanguinity

    Autosomal recessive disease would not explain the milder disease in the mother as a heteroplasmic carrier.

  • C. Mitochondrial DNA mutation inherited maternally, with heteroplasmy (correct)

    Correct. Mitochondrial DNA is inherited from the mother, and heteroplasmy explains the variable expression.

  • D. X-linked recessive with transmission through carrier mothers to sons only

    X-linked recessive inheritance affects mainly males through carrier mothers, but in MELAS both sexes are affected.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is C, Mitochondrial DNA mutation inherited maternally, with heteroplasmy. MELAS (mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes) is most often due to a point mutation in the mitochondrial tRNA-Leu gene.

Why is "Autosomal dominant with full penetrance" not the answer?

Autosomal dominant disease would pass from either parent, and affected fathers would transmit it.

Why is "Autosomal recessive with consanguinity" not the answer?

Autosomal recessive disease would not explain the milder disease in the mother as a heteroplasmic carrier.

Why is "X-linked recessive with transmission through carrier mothers to sons…" not the answer?

X-linked recessive inheritance affects mainly males through carrier mothers, but in MELAS both sexes are affected.

What concept does this question test?

It tests Molecular biology and genetics: MELAS in Biochemistry. Mitochondrial DNA is passed only from the mother to all her children, and the proportion of mutant mitochondria varies between tissues (heteroplasmy), which explains variable severity. Ragged red fibres reflect proliferation of abnormal mitochondria.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Molecular biology and genetics: MELAS?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.