Kaplan USMLE Step 1: A study of blood supply to the brain

If you’re preparing for the United States Medical Licensing Examination® (USMLE®) Step 1 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 researcher is studying the developmental origins of the arteries that supply blood to the brain. The investigation involves injecting contrast media into the hearts of fetal pigs and studying the arteries that develop in the pharyngeal arches. Which of the following pairs of pharyngeal arches gives rise to arteries that contribute to the blood vessels that supply the brain?

A. Pharyngeal arches 1 and 2 

B. Pharyngeal arches 2 and 3 

C. Pharyngeal arches 3 and 4 

D. Pharyngeal arches 4 and 5 

E. Pharyngeal arches 4 and 6 

The correct answer is C.

The third aortic arch, found in arch 3, develops into the common carotid and internal carotid arteries. The fourth aortic arch on the left, found in left arch 4 develops into the adult aortic arch. The fourth aortic arch on the right, found in the right arch 4, develops into the right subclavian artery. The vertebral arteries are branches of the subclavian arteries. The vertebral arteries and the internal carotid arteries provide blood supply to the brain.

Choice A. Pharyngeal arches 3 and 4 give rise to arteries that supply the brain. The first and second aortic arches, found in pharyngeal arches 1 and 2, degenerate and do not give rise to any significant adult artery.