Cytogenetics: 22q11.2 microdeletion: 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 · Cytogenetics: 22q11.2 microdeletion

Sample item

A neonate has hypocalcaemic seizures, truncus arteriosus, a cleft palate and an absent thymic shadow on chest radiograph. Routine G-banded karyotyping is reported as 46,XX. What is the most appropriate next investigation?

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

The answer and the reasoning

C. FISH using a probe for chromosome 22q11.2

The picture is DiGeorge (22q11.2 deletion) syndrome with hypoparathyroidism, conotruncal heart disease, thymic hypoplasia and palatal anomalies. The deletion of about 3 Mb, including TBX1, is submicroscopic and is missed by routine karyotype, so a FISH probe for the region (or chromosomal microarray) detects it. T-cell immunodeficiency means irradiated blood products are needed.

Option by option

  • A. Repeat G-banding on a second tissue

    Repeating G-banding will not reveal a deletion too small to be seen microscopically.

  • B. Sequencing of the CFTR gene

    CFTR sequencing is for cystic fibrosis and does not explain this combination of findings.

  • C. FISH using a probe for chromosome 22q11.2 (correct)

    Submicroscopic deletions are detected by targeted FISH for 22q11.2.

  • D. Southern blot for trinucleotide repeat expansion

    Trinucleotide repeat testing applies to disorders such as fragile X and Huntington disease.

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, FISH using a probe for chromosome 22q11.2. The picture is DiGeorge (22q11.

Why is "Repeat G-banding on a second tissue" not the answer?

Repeating G-banding will not reveal a deletion too small to be seen microscopically.

Why is "Sequencing of the CFTR gene" not the answer?

CFTR sequencing is for cystic fibrosis and does not explain this combination of findings.

Why is "Southern blot for trinucleotide repeat expansion" not the answer?

Trinucleotide repeat testing applies to disorders such as fragile X and Huntington disease.

What concept does this question test?

It tests Cytogenetics: 22q11.2 microdeletion in Biochemistry. 2 deletion) syndrome with hypoparathyroidism, conotruncal heart disease, thymic hypoplasia and palatal anomalies. The deletion of about 3 Mb, including TBX1, is submicroscopic and is missed by routine karyotype, so a FISH probe for the region (or chromosomal microarray) detects it. T-cell immunodeficiency means irradiated blood products are needed.

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 Cytogenetics: 22q11.2 microdeletion?

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.