Medical genetics: achondroplasia: practice question

A Anatomy 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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Anatomy · Medical genetics: achondroplasia

Sample item

A short-limbed child with a large head and a trident hand is born to parents of average height. Which statement best explains the cause?

AX-linked dominant mutation of COL2A1BNew gain-of-function mutation of FGFR3CLoss-of-function mutation of FGFR3 inherited as a recessiveDMitochondrial DNA deletion

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

The answer and the reasoning

B. New gain-of-function mutation of FGFR3

Achondroplasia is autosomal dominant and caused by a gain-of-function FGFR3 mutation that inhibits cartilage growth. Most cases are new mutations, often from the paternal germline in older fathers, which is why the parents are of average height.

Option by option

A. X-linked dominant mutation of COL2A1
COL2A1 mutations cause other skeletal dysplasias, and the usual cause here is autosomal FGFR3.
B. New gain-of-function mutation of FGFR3 (correct)
Correct. A de novo gain-of-function FGFR3 mutation explains average-height parents.
C. Loss-of-function mutation of FGFR3 inherited as a recessive
The FGFR3 change is a gain of function inherited dominantly, not a recessive loss of function.
D. Mitochondrial DNA deletion
Mitochondrial deletions do not cause skeletal dysplasia of this type.

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 Anatomy question

What is the answer to this Anatomy question?

The best answer is B, New gain-of-function mutation of FGFR3. Achondroplasia is autosomal dominant and caused by a gain-of-function FGFR3 mutation that inhibits cartilage growth.

Why is "X-linked dominant mutation of COL2A1" not the answer?

COL2A1 mutations cause other skeletal dysplasias, and the usual cause here is autosomal FGFR3.

Why is "Loss-of-function mutation of FGFR3 inherited as a recessive" not the answer?

The FGFR3 change is a gain of function inherited dominantly, not a recessive loss of function.

Why is "Mitochondrial DNA deletion" not the answer?

Mitochondrial deletions do not cause skeletal dysplasia of this type.

What concept does this question test?

It tests Medical genetics: achondroplasia in Anatomy. Most cases are new mutations, often from the paternal germline in older fathers, which is why the parents are of average height.

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 Medical genetics: achondroplasia?

Learn every nerve by its route, its muscles and what a lesion looks like at the bedside. That one habit answers a large share of questions. Then use the flashcards and the other Anatomy 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.