- Which of the following molecular techniques is most suitable for rapidly detecting this specific known mutation?
Answer: Allele-specific oligonucleotide (ASO) hybridization
Allele-specific oligonucleotide (ASO) hybridization uses probes that bind specifically to either the wild-type or the mutant DNA sequence. This makes it an ideal, rapid, and cost-effective method for screening for a known, common point mutation or small deletion like ΔF508. Whole genome sequencing is unnecessarily broad and expensive for this purpose. Northern blotting analyzes RNA, not the DNA mutation itself. Chromosomal microarray detects copy number variants, not small deletions within a gene.
- Which of the following molecular techniques is most commonly used to detect maternal cell contamination in a fetal DNA sample?
Answer: Analysis of polymorphic short tandem repeats (STRs)
Analysis of highly variable short tandem repeats (STRs) is a form of DNA fingerprinting. The fetal sample should contain one allele from the mother and one from the father at each STR locus. If the fetal sample's STR profile shows more than one maternal allele, or a signal intensity ratio that is skewed towards the maternal alleles, it indicates maternal cell contamination. The other methods are not suitable for this specific quality control purpose.
- Which of the following genetic tests offers the highest diagnostic yield in this scenario?
Answer: Whole exome sequencing (WES)
In cases of a complex, undiagnosed suspected genetic disorder where targeted tests have been negative, whole exome sequencing (WES) is the most appropriate next step. WES analyzes the protein-coding regions of all known genes (~20,000) simultaneously, allowing for the identification of mutations in rare or novel disease genes without an a priori hypothesis. The other options are too targeted for a patient with an unknown diagnosis.
- Which of the following genetic tests is the most specific and sensitive method for detecting this microdeletion?
Answer: Fluorescence in situ hybridization (FISH)
DiGeorge syndrome is a classic microdeletion syndrome. The deletion at 22q11.2 is typically too small to be visualized on a standard G-banded karyotype. Fluorescence in situ hybridization (FISH) uses a DNA probe specific to the 22q11.2 region, which will fail to bind if the region is deleted, making it the gold standard for diagnosis. Serum calcium is a biochemical finding, not a genetic test. Y-chromosome analysis is irrelevant.
- Which of the following techniques is most suitable for measuring the level of HER2 mRNA?
Answer: Quantitative reverse transcription PCR (qRT-PCR)
Quantitative reverse transcription PCR (qRT-PCR) is the gold standard for quantifying levels of a specific mRNA. The technique first uses reverse transcriptase to convert mRNA into complementary DNA (cDNA), and then uses PCR to amplify and quantify the amount of cDNA, which directly correlates with the initial amount of mRNA. FISH assesses gene copy number (DNA). Karyotyping looks at whole chromosomes. Western blotting measures protein levels.
- Which of the following diagnostic tests is most effective for confirming the diagnosis by detecting the abnormal DNA methylation pattern characteristic of Angelman syndrome, regardless of whether it is caused by a deletion or uniparental disomy?
Answer: Methylation-specific PCR
Angelman and Prader-Willi syndromes are classic imprinting disorders. The genes in this region are differentially methylated depending on their parental origin. Regardless of the underlying cause (maternal deletion, paternal uniparental disomy, or an imprinting center defect), Angelman syndrome results in an abnormal, exclusively paternal methylation pattern. Methylation-specific PCR is designed to detect these parent-of-origin-specific methylation patterns and is therefore a key diagnostic test.
- Which of the following molecular diagnostic techniques is specifically designed to detect deletions and duplications of multiple exons simultaneously and is a common first-line test for DMD?
Answer: Multiplex ligation-dependent probe amplification (MLPA)
Multiplex ligation-dependent probe amplification (MLPA) is a semiquantitative PCR-based method designed to determine the copy number of up to 50 different DNA sequences in a single reaction. It is highly effective for detecting exon-level deletions and duplications, making it a standard first-line test for disorders like DMD where such mutations are common. Sanger sequencing is better for point mutations, and karyotyping cannot resolve intragenic changes. Northern blotting assesses mRNA levels.
- The fundamental principle that allows this DNA 'probe' to bind specifically to its target sequence on the chromosome is known as which of the following?
Answer: Hybridization
Hybridization is the process by which a single-stranded nucleic acid molecule (DNA or RNA) anneals to its complementary sequence through hydrogen bonds between the bases. This principle of specific base pairing (A with T, G with C) is the foundation for techniques like FISH, Southern blotting, Northern blotting, and microarrays, where a labeled probe is used to detect a target sequence.
- Analysis of which of the following would provide the most direct assessment of the X-inactivation pattern in this patient?
Answer: DNA methylation status of X-linked gene promoters
X-inactivation is an epigenetic process that involves transcriptional silencing of one X chromosome through DNA methylation. The inactive X chromosome undergoes extensive hypermethylation of CpG islands in gene promoter regions. Analyzing the methylation patterns of X-linked genes can determine which X chromosome (normal vs. mutant allele) is preferentially inactivated in different cell populations, explaining why this carrier remains asymptomatic despite having one defective copy of the coagulation factor gene.
- Which of the following laboratory techniques would be most appropriate to determine if the dystrophin protein is absent in the patient's muscle tissue?
Answer: Western blot
The question specifically asks about the presence or absence of a protein. Western blotting is the technique used to separate proteins by size via gel electrophoresis, transfer them to a membrane, and then use a specific antibody (in this case, anti-dystrophin) to detect the protein of interest. Absence of a band would confirm the diagnosis. Southern blot analyzes DNA, Northern blot analyzes RNA, and chromosomal microarray analyzes for DNA copy number variations.
- The normal karyotype result rules out a large structural or numerical chromosomal abnormality but cannot detect which of the following?
Answer: A single-gene point mutation
A karyotype provides a low-resolution, microscopic view of the chromosomes. It is excellent for detecting aneuploidy (abnormal chromosome number), such as trisomy 18, and large structural changes like major deletions or translocations. However, its resolution is far too low to detect changes within a single gene, such as a point mutation, a small deletion, or an insertion, which require molecular techniques like DNA sequencing.
- Which of the following tests offers the highest resolution for detecting genome-wide copy number variants, such as microdeletions and microduplications?
Answer: Chromosomal microarray analysis (CMA)
Chromosomal microarray analysis (CMA) is the recommended first-tier genetic test for individuals with unexplained developmental delay or congenital anomalies. It can detect copy number variants (gains or losses of DNA segments) at a much higher resolution than a karyotype, identifying submicroscopic abnormalities. FISH is a targeted test for a specific region, not a genome-wide scan. Southern blotting is not used for this purpose. Repeating the low-resolution karyotype would be unhelpful.
- Compared to a sample from a healthy individual, the Western blot from this patient is most likely to show a protein band that is:
Answer: Shifted to a lower apparent molecular weight
Becker muscular dystrophy results from mutations that allow for the production of a smaller, internally deleted, but still partially functional dystrophin protein. On a Western blot, which separates proteins by size, this truncated protein will migrate further down the gel than the full-length protein from a healthy control. This results in a band at a lower apparent molecular weight. An absent band is characteristic of Duchenne muscular dystrophy.
- Molecular analysis of the FMR1 gene promoter in this patient is most likely to show which finding?
Answer: Extensive DNA hypermethylation
Fragile X syndrome is caused by a large expansion of a CGG trinucleotide repeat in the 5' untranslated region of the FMR1 gene. When the number of repeats exceeds 200 (a full mutation), the CpG islands in the promoter region become hypermethylated. This methylation silences the gene, preventing transcription of FMR1 mRNA and production of the FMRP protein. Histone acetylation is associated with gene activation, not silencing.
- Which of the following is the most appropriate recommendation for this patient?
Answer: Undergo diagnostic testing via amniocentesis or CVS
Non-invasive prenatal testing (NIPT) is a highly accurate screening test, but it is not diagnostic. A high-risk result must be confirmed with a diagnostic test that directly analyzes fetal cells. The options for definitive diagnosis are invasive procedures such as chorionic villus sampling (CVS) in the first trimester or amniocentesis in the second trimester. Clinical decisions, such as pregnancy termination, should only be made after a confirmed diagnostic result.
- This result is most consistent with a diagnosis of Down syndrome due to which of the following mechanisms?
Answer: Meiotic nondisjunction
The karyotype 47,XX,+21 indicates that the child, a female, has 47 chromosomes in total, with an extra copy of chromosome 21. This is the classic karyotype for trisomy 21 (Down syndrome). The most common cause (approximately 95% of cases) is the failure of chromosome 21 to segregate properly during meiosis in a parent (meiotic nondisjunction), leading to a gamete with an extra chromosome 21. Robertsonian translocation would result in 46 chromosomes. Mosaicism would show two cell lines. Uniparental disomy would result in 46 chromosomes.
- Given that the hepatitis C virus is an RNA virus, which of the following is the most sensitive and specific laboratory test to detect and quantify the viral load?
Answer: Quantitative reverse transcription PCR (qRT-PCR)
Quantitative reverse transcription PCR (qRT-PCR) is the standard method for detecting and quantifying RNA viruses like hepatitis C. The viral RNA is first converted to cDNA by reverse transcriptase, and then the cDNA is amplified and quantified by PCR. This method is extremely sensitive and specific. ELISA for antibodies indicates exposure but not necessarily active infection. Viral culture is difficult and slow for HCV. Southern blot detects DNA, not RNA.
- Which of the following components must also be included in the reaction mixture to serve as the building blocks for the newly synthesized DNA strands?
Answer: Deoxyribonucleoside triphosphates (dNTPs)
PCR is an in vitro method for DNA synthesis. The DNA polymerase enzyme requires a supply of monomer units to build the new DNA strands. These building blocks are the four deoxyribonucleoside triphosphates: dATP, dCTP, dGTP, and dTTP, collectively known as dNTPs. Reverse transcriptase synthesizes DNA from an RNA template. DNA ligase joins DNA fragments. RNA polymerase synthesizes RNA from a DNA template.
- This research method, which tracks the co-segregation of a genetic marker and a disease phenotype within a family, is best described as which of the following?
Answer: Linkage analysis
Linkage analysis is a method used to map a disease-causing gene to a specific chromosomal region by studying its inheritance pattern alongside known genetic markers in large families. Markers that are physically close to the disease gene on a chromosome tend to be inherited together (i.e., they are 'linked') and will not assort independently. GWAS is a population-based study to find common variants associated with common diseases, not for mapping rare single-gene disorders in families.
- In a CML cell containing the classic t(9;22) translocation, which of the following signal patterns is expected?
Answer: One red signal, one green signal, and two yellow (fusion) signals
In a diploid CML cell, there is one normal chromosome 9 (one green signal), one normal chromosome 22 (one red signal), the derivative chromosome 9 (containing a piece of 22), and the derivative chromosome 22 (Philadelphia chromosome, containing a piece of 9). The BCR-ABL fusion on the Philadelphia chromosome brings a red and green probe close together, creating a yellow/fusion signal. The reciprocal ABL-BCR fusion on the derivative chromosome 9 also creates a fusion signal. Therefore, the pattern is one separate red, one separate green, and two fusion signals.
