Chromosomal abnormalities · USMLE Step 1

A newborn female has ambiguous genitalia, cleft palate, and holoprosencephaly. Genetic testing confirms trisomy 13. What is the chromosomal basis?

  1. Three copies of chromosome 18
  2. Three copies of chromosome 13
  3. Deletion on chromosome 15q
  4. Uniparental disomy of chromosome 7
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Correct answer: Three copies of chromosome 13

Patau syndrome is trisomy 13, caused by an extra copy of chromosome 13. It leads to severe midline defects. Other options correspond to different conditions.

Difficulty: Medium Question 13 of 20

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