In subclavian steal syndrome, what occurs due to proximal subclavian artery stenosis?

In subclavian steal syndrome, proximal subclavian artery stenosis leads to a decrease in blood flow distal to the stenosis. To compensate for this reduced blood flow, the body redirects blood from the vertebral artery, which is typically a branch of the subclavian artery. This results in retrograde (reverse) flow in the vertebral artery, causing blood to flow back toward the subclavian artery rather than toward the brain.

This compensatory mechanism is a classic feature of subclavian steal syndrome, as the body attempts to maintain perfusion to the affected regions at the expense of cerebral blood flow. Therefore, the correct answer highlights how the stenosis affects blood flow dynamics, particularly the shift in the direction of flow in the vertebral artery.