Chromosomal abnormalities Practice Questions
20 free Chromosomal abnormalities practice questions for the USMLE Step 1. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
A newborn presents with hypotonia, upslanting palpebral fissures, a single palmar crease, and a congenital heart defect. Cytogenetic analysis confirms trisomy 21. Which mechanism most commonly leads to this chromosomal abnormality?
- A Robertsonian translocation
- B Mitotic nondisjunction
- C Meiotic nondisjunction
- D Chromosomal deletion
Correct answer: Meiotic nondisjunction
Most cases of trisomy 21 are due to meiotic nondisjunction, particularly during maternal meiosis I. It results in an extra copy of chromosome 21. Other mechanisms, like Robertsonian translocation, account for a smaller proportion of cases.
A 3-year-old child has severe intellectual disability, microcephaly, and a high-pitched crying sound. Karyotype shows a partial deletion on chromosome 5p. Which syndrome is most likely?
- A Cri-du-chat syndrome
- B Williams syndrome
- C Angelman syndrome
- D Wolf-Hirschhorn syndrome
Correct answer: Cri-du-chat syndrome
Cri-du-chat syndrome is caused by a deletion on chromosome 5p, leading to a characteristic high-pitched cry. Intellectual disability and microcephaly are also typical features. The other conditions involve different chromosomal abnormalities.
Which chromosomal abnormality results from a reciprocal translocation between the long arms of chromosomes 14 and 21?
- A Balanced reciprocal translocation
- B Robertsonian translocation
- C Isochromosome formation
- D Ring chromosome formation
Correct answer: Robertsonian translocation
Robertsonian translocation involves the fusion of two acrocentric chromosomes, such as chromosomes 14 and 21. It is a common cause of familial Down syndrome. Balanced translocations do not involve this specific fusion pattern.
A patient with short stature, streak ovaries, webbed neck, and coarctation of the aorta likely has which chromosomal finding?
- A 47,XXX
- B 47,XXY
- C 45,X
- D 46,XY deletion
Correct answer: 45,X
Turner syndrome is characterized by a 45,X karyotype. This results in gonadal dysgenesis and physical findings such as webbed neck and cardiac anomalies. The other karyotypes correspond to different syndromes.
A teenager with tall stature, small testes, gynecomastia, and reduced facial hair undergoes genetic testing. The likely result is:
- A 47,XYY
- B 47,XXY
- C 45,X
- D 46,XX deletion
Correct answer: 47,XXY
Klinefelter syndrome (47,XXY) presents with tall stature, small testes, and gynecomastia due to hypogonadism. The extra X chromosome leads to impaired sexual development. XYY males generally have normal sexual development.
Microarray testing on a child with intellectual disability and elfin facies reveals a deletion on chromosome 7q11.23. Which condition is this?
- A DiGeorge syndrome
- B Prader-Willi syndrome
- C Williams syndrome
- D Angelman syndrome
Correct answer: Williams syndrome
Williams syndrome results from a microdeletion on chromosome 7q11.23, affecting elastin. Elfin facies and cognitive deficits are characteristic. The other disorders involve different chromosomal loci.
A newborn with congenital heart disease and thymic aplasia is diagnosed with 22q11 deletion. Which mechanism caused this abnormality?
- A Uniparental disomy of chromosome 22
- B Microdeletion via non-allelic homologous recombination
- C Balanced reciprocal translocation event
- D Isochromosome formation of chromosome 22
Correct answer: Microdeletion via non-allelic homologous recombination
22q11 deletions often occur due to non-allelic homologous recombination, causing microdeletions. This leads to DiGeorge syndrome features such as thymic aplasia. Uniparental disomy does not cause this syndrome.
A child with severe hypotonia, obesity, hyperphagia, and small hands/feet likely has which chromosomal mechanism causing the disease?
- A Maternal deletion on chromosome 15q
- B Paternal deletion on chromosome 15q
- C Uniparental disomy maternal
- D Trisomy 15 mosaicism
Correct answer: Paternal deletion on chromosome 15q
Prader-Willi syndrome results from loss of paternal genes on 15q, either via deletion or maternal uniparental disomy. Symptoms include obesity and hypotonia. Maternal deletions cause Angelman syndrome.
A woman with multiple miscarriages is found to carry a balanced Robertsonian translocation. What is her chromosomal composition?
- A 45 chromosomes with one fused chromosome
- B 47 chromosomes with one extra chromosome
- C Normal chromosome number but structural rearrangement
- D Both homologs of a chromosome inherited from one parent
Correct answer: 45 chromosomes with one fused chromosome
Robertsonian translocation carriers have 45 chromosomes due to fusion of two acrocentric chromosomes. They are usually phenotypically normal but have reproductive risks. The other options describe different chromosomal outcomes.
A child presents with hypertelorism, cleft lip/palate, seizures, and a deletion on chromosome 4p. Which condition is this?
- A Wolf-Hirschhorn syndrome
- B Edwards syndrome
- C Patau syndrome
- D Angelman syndrome
Correct answer: Wolf-Hirschhorn syndrome
Wolf-Hirschhorn syndrome is due to a deletion on chromosome 4p. Clinical features include facial abnormalities and seizures. The other syndromes have distinct chromosomal etiologies.
A patient with trisomy 18 presents with overlapping fingers, rocker-bottom feet, and cardiac abnormalities. What meiotic error most commonly causes this condition?
- A Paternal meiosis I nondisjunction
- B Maternal meiosis I nondisjunction
- C Paternal meiosis II nondisjunction
- D Postzygotic mitotic error
Correct answer: Maternal meiosis I nondisjunction
Trisomy 18 (Edwards syndrome) is most often caused by maternal meiosis I nondisjunction. Advanced maternal age is a major risk factor. Postzygotic errors more commonly cause mosaicism.
A karyotype shows 69,XXX in a spontaneously aborted fetus. Which abnormality does this represent?
- A Trisomy
- B Tetraploidy
- C Triploidy
- D Monosomy
Correct answer: Triploidy
Triploidy is characterized by 69 chromosomes, often due to dispermy. It results in early pregnancy loss. Trisomy affects only one chromosome pair, not the entire set.
A newborn female has ambiguous genitalia, cleft palate, and holoprosencephaly. Genetic testing confirms trisomy 13. What is the chromosomal basis?
- A Three copies of chromosome 18
- B Three copies of chromosome 13
- C Deletion on chromosome 15q
- D Uniparental disomy of chromosome 7
Correct answer: Three copies of chromosome 13
Patau syndrome is trisomy 13, caused by an extra copy of chromosome 13. It leads to severe midline defects. Other options correspond to different conditions.
A lab report shows the presence of an isochromosome of the X chromosome. This finding is most associated with which condition?
- A Prader-Willi syndrome
- B Turner syndrome
- C Down syndrome
- D Edwards syndrome
Correct answer: Turner syndrome
Turner syndrome can result from an isochromosome X, where both arms are identical. This leads to monosomy of one arm and duplication of the other. It contributes to the clinical features of Turner syndrome.
Which chromosomal finding is most consistent with chronic myelogenous leukemia (CML)?
- A Deletion of chromosome 7q
- B Translocation t(9;22)
- C Trisomy of chromosome 8
- D Isochromosome of 12p
Correct answer: Translocation t(9;22)
CML is associated with the Philadelphia chromosome, a t(9;22) translocation forming the BCR-ABL fusion gene. This leads to uncontrolled tyrosine kinase activity. Other abnormalities relate to different malignancies.
A newborn with micrognathia, low-set ears, and clenched fists is diagnosed with Edwards syndrome. What karyotype confirms this condition?
- A 47,XY,+13
- B 47,XX,+18
- C 47,XX,+21
- D 45,X
Correct answer: 47,XX,+18
Edwards syndrome is trisomy 18, so the karyotype would show +18. Clinical features include clenched fists and cardiac defects. Other options represent different syndromes.
A child presents with ataxia, seizures, and inappropriate laughter. Genetic testing reveals maternal imprinting defect with paternal uniparental disomy. Which syndrome is most consistent?
- A Prader-Willi syndrome
- B Angelman syndrome
- C Wolf-Hirschhorn syndrome
- D Williams syndrome
Correct answer: Angelman syndrome
Angelman syndrome results from loss of maternal UBE3A expression, often due to paternal uniparental disomy or maternal deletion. Clinical features include ataxia and happy demeanor. Prader-Willi involves the paternal genes on the same locus.
A karyotype reveals mosaicism with two cell lines: 46,XX and 47,XX,+21. Which statement is correct?
- A The child will be entirely free of any clinical features
- B The syndrome arises from an unbalanced chromosomal translocation
- C Severity may be milder than full trisomy 21
- D This karyotype instead produces monosomy X (Turner syndrome)
Correct answer: Severity may be milder than full trisomy 21
Mosaic Down syndrome often presents with milder symptoms because not all cells carry the extra chromosome 21. Mosaicism arises from postzygotic mitotic nondisjunction. Translocation causes a different subtype of Down syndrome, and monosomy X is unrelated.
A fetus has cystic hygroma and hydrops fetalis. Karyotype reveals 45,X. Which mechanism explains the monosomy?
- A Paternal nondisjunction
- B Maternal nondisjunction
- C Mitotic error after fertilization
- D Any of the above
Correct answer: Any of the above
Turner syndrome (45,X) can result from paternal or maternal nondisjunction or a mitotic error. Multiple mechanisms can produce monosomy X. The clinical findings are consistent with Turner syndrome.
A neonate presents with polydactyly, microcephaly, and congenital heart disease. Genetic testing shows meiotic nondisjunction. Which trisomy is most likely?
- A Trisomy 21
- B Trisomy 18
- C Trisomy 13
- D Trisomy 22
Correct answer: Trisomy 13
Trisomy 13 (Patau syndrome) commonly presents with polydactyly and severe craniofacial anomalies. It is caused by meiotic nondisjunction. The other trisomies have different clinical patterns.