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.

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.

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.

The man has a pericentric inversion, which involves a segment of a chromosome that includes the centromere. During meiosis, if a crossover event occurs within the inverted segment, it can lead to the formation of recombinant chromosomes that have a duplication of one end and a deletion of the other. Fertilization with such a gamete results in an unbalanced karyotype and is often lethal or causes severe congenital anomalies.

The SRY (Sex-determining Region on Y) gene, located on the short arm of the Y chromosome, is the primary genetic switch for male development. It encodes a transcription factor that initiates the differentiation of the gonads into testes. A deletion or mutation in the SRY gene in a 46,XY individual leads to gonadal dysgenesis, where the gonads fail to develop properly into testes. Without functional testes, there is insufficient testosterone and anti-Müllerian hormone production, resulting in the development of female or ambiguous external genitalia.

This patient presents with features of DiGeorge syndrome (CATCH-22: Cardiac defects, Abnormal facies, Thymic hypoplasia, Cleft palate, Hypocalcemia). This syndrome is caused by a microdeletion at chromosome 22q11.2. The developmental failure of the third and fourth pharyngeal pouches is responsible for the thymic hypoplasia (leading to T-cell immunodeficiency and recurrent infections) and parathyroid hypoplasia (leading to hypocalcemia and tetany).

The child inherited two different homologous chromosomes from the mother. This means that the mother's homologous pair of chromosome 21 failed to separate during meiosis I. This failure of separation (nondisjunction) resulted in a gamete containing both homologous chromosomes. If nondisjunction had occurred in meiosis II, the gamete would have contained two identical sister chromatids.

The infant's features are classic for Down syndrome (Trisomy 21). The most common cause of trisomy 21, accounting for over 90% of cases, is nondisjunction of chromosome 21 during maternal meiosis I. This risk increases significantly with advanced maternal age due to the prolonged arrest of primary oocytes in prophase I.

A Barr body is an inactivated X chromosome. The number of Barr bodies in a cell is equal to the number of X chromosomes minus one (n-1 rule). A normal female (46,XX) has one Barr body. A patient with two Barr bodies per cell would have three X chromosomes. The karyotype is therefore 47,XXX (Triple X syndrome). Many individuals with this condition have a normal phenotype or mild features like tall stature and learning disabilities.

The mother has a balanced Robertsonian translocation involving chromosomes 14 and 21. During meiosis, she can produce gametes that result in an offspring with translocation Down syndrome. Such a child would inherit her translocated chromosome (der(14;21)) as well as a normal chromosome 21 from her and a normal chromosome 14 and 21 from the father. This results in 46 total chromosomes but three effective copies of the long arm of chromosome 21, leading to the phenotype of Down syndrome. The karyotype is written as 46,XX,der(14;21)(q10;q10),+21.