Molecular biology and genetics: single large mtDNA deletion: 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: single large mtDNA deletion

Sample item

A 20-year-old has ptosis, ophthalmoplegia, pigmentary retinopathy and heart block. Muscle biopsy shows ragged red fibres and a single large deletion of mitochondrial DNA. Neither parent is affected and there is no family history. Why is the disease sporadic?

ASingle large mtDNA deletions usually arise de novo and are rarely inheritedBIt is X-linked recessiveCIt is due to paternal imprintingDIt is autosomal recessive with unaffected carrier parents

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

The answer and the reasoning

A. Single large mtDNA deletions usually arise de novo and are rarely inherited

Kearns-Sayre syndrome is usually caused by a single large mtDNA deletion that arises sporadically in the oocyte or early embryo and is rarely transmitted. Point mutations, in contrast, show maternal inheritance. Heteroplasmy and the threshold effect determine severity.

Option by option

A. Single large mtDNA deletions usually arise de novo and are rarely inherited (correct)
Correct. A single large deletion arises de novo and is usually not transmitted.
B. It is X-linked recessive
Mitochondrial genes are not on the X chromosome.
C. It is due to paternal imprinting
Imprinting does not explain mtDNA deletions.
D. It is autosomal recessive with unaffected carrier parents
Autosomal recessive inheritance would be due to a nuclear gene, not an mtDNA deletion.

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.

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is A, Single large mtDNA deletions usually arise de novo and are rarely inherited. Kearns-Sayre syndrome is usually caused by a single large mtDNA deletion that arises sporadically in the oocyte or early embryo and is rarely transmitted.

Why is "It is X-linked recessive" not the answer?

Mitochondrial genes are not on the X chromosome.

Why is "It is due to paternal imprinting" not the answer?

Imprinting does not explain mtDNA deletions.

Why is "It is autosomal recessive with unaffected carrier parents" not the answer?

Autosomal recessive inheritance would be due to a nuclear gene, not an mtDNA deletion.

What concept does this question test?

It tests Molecular biology and genetics: single large mtDNA deletion in Biochemistry. Point mutations, in contrast, show maternal inheritance. Heteroplasmy and the threshold effect determine severity.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. 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: single large mtDNA deletion?

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.