Kaplan USMLE Step 2: What does fetal ultrasound show?

If you’re preparing for the United States Medical Licensing Examination® (USMLE®) Step 2 exam, you might want to know which questions are most often missed by test-prep takers. Check out this example from Kaplan Medical, and read an expert explanation of the answer. Also check out all posts in this series.

A 39-year-old woman, gravida 3, para 2, at 20 weeks' gestation, comes to the physician to initiate prenatal care. She has had no bleeding or pain. Fetal movement has been normal. Her two prior pregnancies ended with spontaneous vaginal deliveries of normal infants. The patient has no chronic medical problems and is taking no medications. Blood pressure is 120/78 mm Hg. The fetal heart rate is 142/min. An ultrasound shows a male infant with a hypoplastic nasal bone, duodenal atresia, and a large ventricular septal defect.

Which of the following is the most likely etiology of this fetus's condition?

A. Nondisjunction during meiosis

B. Nondisjunction during mitosis

C. Partial deletion of chromosome 5

D. Ring chromosome X formation

E. Trinucleotide repeat expansion

The correct answer is A.

Nondisjunction during meiosis is the most common etiology of trisomy 21 which can present at 20 weeks' gestation with a fetal ultrasound showing hypoplastic nasal bone, duodenal atresia, and a large ventricular septal defect.

Choice B. Nondisjunction during mitosis causes mosaicism, an uncommon cause of trisomy 21. Trisomy 21 mosaics typically present with milder disease. This fetus's disease appears severe given the sonographic findings.