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Which statement correctly explains the etiology of Down syndrome?

  • A. There is an extra chromosome on the 21st pair.
  • B. There is a missing chromosome on the 21st pair.
  • C. There are two pairs of the 21st chromosome.
  • D. The 21st chromosome pair is missing.
Correct Answer: A

Rationale: The correct answer is A: 'There is an extra chromosome on the 21st pair.' Down syndrome is caused by the presence of an extra copy of chromosome 21, known as trisomy 21. This additional genetic material leads to the characteristics associated with Down syndrome. Choices B, C, and D are incorrect because Down syndrome is not due to a missing chromosome or having two pairs of the 21st chromosome; it results from the presence of an extra chromosome on the 21st pair.