Chromosomal Disorders
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USMLE Step 1 › Chromosomal Disorders
A 2-day-old newborn with trisomy 21 is evaluated for gastrointestinal obstruction. Abdominal radiograph shows a “double bubble” sign. Which congenital anomaly best explains this imaging finding in this chromosomal disorder?
Malrotation with volvulus due to abnormal midgut rotation
Hirschsprung disease due to absent Meissner plexus
Duodenal atresia causing proximal intestinal obstruction
Pyloric stenosis causing nonbilious projectile vomiting
Meconium ileus due to CFTR mutation and thick secretions
Explanation
This question tests understanding of chromosomal disorders and their genetic mechanisms on the USMLE Step 1. Chromosomal disorders arise from numerical or structural abnormalities, such as trisomies or deletions, leading to various phenotypic manifestations. In this vignette, the patient's symptoms and lab findings suggest Down syndrome, supported by the karyotype showing trisomy 21. The correct choice is A, as it aligns with the clinical and genetic data provided, demonstrating understanding of chromosomal mechanisms. Choice B fails because it inaccurately associates symptoms with pyloric stenosis, a common error when phenotypic traits overlap. To prepare, focus on understanding the genetic basis of common chromosomal disorders and recognize the clinical presentations they typically cause. Practice analyzing karyotypes and relating lab data to clinical findings.
A 1-month-old infant has a high-pitched cry, microcephaly, and hypotonia. Karyotype shows 46,XY,del(5)(p15). The parents ask what type of chromosomal change this represents compared with trisomy 21. Which statement best explains the difference?
This disorder is due to mitochondrial inheritance, whereas trisomy 21 is autosomal recessive
This disorder is due to balanced translocation, whereas trisomy 21 is always balanced
This disorder is due to monosomy X, whereas trisomy 21 is due to sex chromosome gain
This disorder is due to a structural deletion, whereas trisomy 21 is usually a numerical gain
This disorder is due to duplication of 5p, whereas trisomy 21 is due to deletion
Explanation
This question tests understanding of chromosomal disorders and their genetic mechanisms on the USMLE Step 1. Chromosomal disorders arise from numerical or structural abnormalities, such as trisomies or deletions, leading to various phenotypic manifestations. In this vignette, the patient's symptoms and lab findings suggest cri-du-chat syndrome, supported by the karyotype showing 46,XY,del(5)(p15). The correct choice is A, as it aligns with the clinical and genetic data provided, demonstrating understanding of chromosomal mechanisms. Choice B fails because it inaccurately associates symptoms with monosomy X, a common error when phenotypic traits overlap. To prepare, focus on understanding the genetic basis of common chromosomal disorders and recognize the clinical presentations they typically cause. Practice analyzing karyotypes and relating lab data to clinical findings.
A 4-day-old newborn with hypotonia and facial features consistent with Down syndrome undergoes chromosomal testing. Karyotype shows 46,XX with three copies of chromosome 21 material due to a translocation. The genetic counselor asks about parental testing. Which parental finding would most increase recurrence risk in future pregnancies?
One parent has a de novo 5p deletion with cat-like cry history
One parent has 47,XYY with normal spermatogenesis
One parent has mosaic 45,X/46,XX with normal fertility
One parent is a balanced Robertsonian translocation carrier involving chromosome 21
One parent has 47,XXY with infertility and small testes
Explanation
This question tests understanding of chromosomal disorders and their genetic mechanisms on the USMLE Step 1. Chromosomal disorders arise from numerical or structural abnormalities, such as trisomies or deletions, leading to various phenotypic manifestations. In this vignette, the patient's symptoms and lab findings suggest Down syndrome due to translocation, supported by the karyotype showing 46,XX with three copies of chromosome 21 material due to a translocation. The correct choice is A, as it aligns with the clinical and genetic data provided, demonstrating understanding of chromosomal mechanisms. Choice B fails because it inaccurately associates symptoms with mosaic Turner syndrome, a common error when phenotypic traits overlap. To prepare, focus on understanding the genetic basis of common chromosomal disorders and recognize the clinical presentations they typically cause. Practice analyzing karyotypes and relating lab data to clinical findings.
A 1-week-old male newborn has hypotonia and mild respiratory distress. He was born at 37 weeks to a 36-year-old mother. Physical exam shows upslanting palpebral fissures, epicanthal folds, flat nasal bridge, and a single palmar crease. Cardiac exam reveals a harsh holosystolic murmur. Echocardiogram shows an atrioventricular septal defect. CBC shows WBC 28.0 $×10^3$/µL with circulating blasts; hemoglobin 16.8 g/dL; platelets 95 $×10^3$/µL. Karyotype confirms trisomy 21 (47,XY,+21). The neonatologist explains the most likely hematologic complication associated with this chromosomal disorder.
Increased risk of chronic myelogenous leukemia due to t(9;22)
Polycythemia vera due to JAK2 mutation from extra X chromosome
Increased risk of acute lymphoblastic leukemia and transient myeloproliferation
Aplastic anemia due to bone marrow failure from monosomy X
Severe neutropenia due to 5p deletion affecting granulopoiesis
Explanation
This question tests understanding of chromosomal disorders and their genetic mechanisms on the USMLE Step 1. Chromosomal disorders arise from numerical or structural abnormalities, such as trisomies or deletions, leading to various phenotypic manifestations. In this vignette, the patient's symptoms and lab findings suggest Down syndrome, supported by the karyotype showing trisomy 21 (47,XY,+21). The correct choice is A, as it aligns with the clinical and genetic data provided, demonstrating understanding of chromosomal mechanisms. Choice B fails because it inaccurately associates symptoms with chronic myelogenous leukemia, a common error when phenotypic traits overlap. To prepare, focus on understanding the genetic basis of common chromosomal disorders and recognize the clinical presentations they typically cause. Practice analyzing karyotypes and relating lab data to clinical findings.