Anatomical variants · USMLE Step 1

A CT scan reveals a right-sided aortic arch in an asymptomatic adult. This variant most commonly results from persistence of which embryologic structure?

  1. Right fourth aortic arch
  2. Left fourth aortic arch
  3. Right sixth aortic arch
  4. Left sixth aortic arch
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Correct answer: Right fourth aortic arch

A right-sided aortic arch arises from persistence of the right fourth aortic arch with regression of the left. It is often asymptomatic but may be associated with vascular rings.

Difficulty: Medium Question 2 of 20

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