Chromosomal abnormalities · USMLE Step 1

A neonate presents with polydactyly, microcephaly, and congenital heart disease. Genetic testing shows meiotic nondisjunction. Which trisomy is most likely?

  1. Trisomy 21
  2. Trisomy 18
  3. Trisomy 13
  4. Trisomy 22
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Correct answer: Trisomy 13

Trisomy 13 (Patau syndrome) commonly presents with polydactyly and severe craniofacial anomalies. It is caused by meiotic nondisjunction. The other trisomies have different clinical patterns.

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