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USMLE Step 1 Questions
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A 1-week-old newborn has microcephaly, intrauterine growth retardation, widely spaced eyes, and a cat-like cry. Which of the following karyotypes is most likely?
Correct Answer: C. 46,XX,del(5)(p12)
All the described abnormalities point to Cri-du-Chat syndrome, uniquely characterized by a ‘crying of the cat’ like vocalization made by affected infants. Cri-du-chat results from deletion of part of the short arm of chromosome 5, and spares the normal diploid number. This answer choice is 46,XX,del(5)(p12).
46,XX,del(5)(p12) describes a rare translocation that results in Acute Lymphoblastic Leukemia, not developmental disorders.
46,XX,del(5)(p12) indicates a trisomy in Chromosome 13, resulting in severe mental retardation and growth abnormalities called Patau’s Syndrome. The description above suggests that this is not the best answer.
47,XX,+21 describes Trisomy in chr 21, which is classically Down’s Syndrome. While Down’s syndrome may result in microcephaly, it would most prominently cause mental retardation.
69,XXY is a total triploidy and usually incompatible with life.
1. QuestionCategory: Genetics
A 1-week-old newborn has microcephaly, intrauterine growth retardation, widely spaced eyes, and a cat-like cry. Which of the following karyotypes is most likely?Correct