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D. Triploidy in amniotic fluid - 23%
Triploidy describes the presence of three sets of chromosomes. Like trisomy, triploidy is a type of chromosomal aberration that can be detected during prenatal screening. This patient's first-trimester screening results are suggestive of trisomy 21, rather than a triploidy. In addition, although amniocentesis is a diagnostic test used to confirm the diagnosis of chromosomal aberrations during pregnancy, it is performed in the second trimester between 15–20 weeks' gestation and would not be appropriate in this patient at 12 weeks' gestation.
E. Decreased inhibin A in maternal serum - 7%
Inhibin A levels in maternal serum are typically measured as part of the quadruple screening in the second trimester (specifically between 15–20 weeks' gestation). However, inhibin A levels cannot confirm the presence of a chromosomal aberration. Furthermore, an increased rather than a decreased inhibin A level is consistent with trisomy 21.
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Case #17
Subject: Embryology
A 2400-g (5.29-lb) male newborn is delivered at term to a 26-year-old woman. Physical examination shows a sloping forehead, a flat nasal bridge, increased interocular distance, low-set ears, and a protruding tongue. There is a single palmar crease and an increased gap between the first and second toe. The abdomen is distended. An x-ray of the abdomen shows two large air-filled spaces in the upper quadrant. Karyotype analysis shows 46 chromosomes in all tested cells. Which of the following is the most likely underlying cause of this patient's findings?
A. Balanced translocation
B. Meiotic nondisjunction
C. Mitotic nondisjunction
D. Unbalanced translocation
E. Uniparental disomy
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Case #17
Subject: Embryology
The correct answer is D [ 45% ]
Unbalanced Robertsonian translocation is the pathomechanism underlying translocation trisomy 21, which accounts for approx. 4% of cases of Down syndrome. Translocation trisomy 21 can occur in offsprings of a translocation carrier with a balanced Robertsonian translocation that involves chromosome 21. Inheritance of a translocation chromosome 21 and a normal chromosome 21 leads to three copies of genetic material from chromosome 21 while maintaining the normal total number of 46 chromosomes.
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