This USMLE Step 1 Question of the Day addresses abnormal lab results in a 17-week pregnant patient. Learn about the interpretation of quadruple test results and their implications.
A 39-year-old woman, gravida 1, para 0, comes to the office for the evaluation of abnormal laboratory results. The patient is at 17 weeks gestation according to her last menstrual period. She used to drink 1 glass of wine on weekends during the first trimester, as she was unaware of the pregnancy. Past medical history is unremarkable. She regularly takes prenatal vitamins, and her immunizations are up to date. Results of the quadruple test are shown:
| Laboratory value | Result |
| Maternal serum alpha fetoprotein | Low |
| Unconjugated estriol | Low |
| β-HCG | High |
| Inhibin A | High |
The fetus is at greatest risk of developing which of the following?
A. Myelomeningocele
B. Omphalocele
C. Fetal alcohol syndrome
D. Duodenal atresiaE. Gastroschisis
Scroll down for the correct answer!
The correct answer to today’s USMLE® Step 1 Question is…
D. Duodenal atresia
Before we get to the Main Explanation, let’s look at the incorrect answer explanations. Skip to the bottom if you want to see the correct answer right away!
Incorrect answer explanations
A. Myelomeningocele
Incorrect: Myelomeningocele results from a neural tube defect and causes increased maternal serum AFP, whereas unconjugated estriol, β-HCG and inhibin A is within normal limits.
B. Omphalocele
Incorrect: Omphalocele can be seen in association with Trisomy 18. The quadruple test shows reduced levels of maternal serum AFP, unconjugated estriol, β-HCG and inhibin A. Neonates with Down syndrome are at slightly elevated risk of developing omphalocele; however, their risk for another gastrointestinal anomaly is greater.
C. Fetal alcohol syndrome
Incorrect: Pregnant women should be advised to avoid alcohol intake during pregnancy. However, 1 glass of wine a week does not significantly increase the risk of fetal alcohol syndrome, and the results of this quadruple test suggest a different diagnosis.
E. Gastroschisis
Incorrect: Gastroschisis results from an abdominal wall defect and causes increased maternal serum and amniotic fluid AFP, whereas unconjugated estriol, β-HCG and inhibin A are within normal limits.
Main Explanation
This pregnant patient’s results from the quadruple test demonstrate an elevated hCG and inhibin A with low AFP and estriol; these findings are most indicative of Down syndrome (trisomy 21). Down syndrome is the most common autosomal trisomy and the most frequent form of intellectual disability caused by a chromosomal aberration. It is characterized by the presence of an extra copy of genetic material on chromosome 21, either in whole (trisomy 21) or in part (such as due to translocations).
A fetus with Down syndrome is at greatest risk of duodenal atresia, which results from the failure of canalization of the duodenum in early gestation. Affected infants present with bilious emesis during the first few days of life and “double bubble” sign (dilated stomach and proximal duodenum) on an abdominal x-ray. Other gastrointestinal abnormalities associated with Down syndrome include Hirschsprung disease, imperforate anus, tracheoesophageal fistula, and celiac disease.
“Double bubble” sign in a patient with duodenal atresia
Reproduced from: Wikimedia Commons
Major Takeaway
Duodenal atresia is the most common gastrointestinal anomaly associated with Down syndrome. Affected infants present with bilious emesis during the first few days of life and “double bubble” sign (dilated stomach and proximal duodenum) on an abdominal x-ray. Hirschsprung disease, imperforate anus, tracheoesophageal fistula, and celiac disease are other common associations of Trisomy 21.
References
- Stoll, C., Dott, B., Alembik, Y., & Roth, M. P. (2015). Associated congenital anomalies among cases with Down syndrome. European journal of medical genetics, 58(12), 674-680.
- Epstein CJ. Down syndrome (Trisomy 21). In: The metabolic and molecular bases of inherited disease, 8th ed, Scriver CR, Beaudet AL, Sly WS, Valle D (Eds), McGraw-Hill, New York 2001. p.1223.
________________________
Want more USMLE® Step 1 practice questions? Try Osmosis by Elsevier today! Access your free trial and find out why millions of current and future clinicians and caregivers love learning with us.
Leave a Reply