What describes the condition of Subclavian Steal?

Subclavian steal syndrome occurs when there is an occlusion or significant stenosis in the subclavian artery, typically proximal to the vertebral artery origin. This condition leads to a reversal of blood flow in the vertebral artery as the body attempts to compensate for reduced blood flow to the arm.

When the subclavian artery is narrowed or blocked, blood flow from the vertebral artery is redirected to supply the affected arm, causing a 'stealing' of blood from the posterior circulation. This situation results in decreased flow to the brain's posterior fossa, possibly leading to symptoms like dizziness or vertigo, particularly with arm activity.

The other options do not accurately depict the mechanism or nature of subclavian steal syndrome. While increased collateral circulation occurs in various ischemic conditions, it is not the defining characteristic of this syndrome. Formation of an arteriovenous fistula and long-term venous obstruction are unrelated vascular conditions that do not explain the specific hemodynamic changes seen in subclavian steal. Thus, the reversal of blood flow in the vertebral artery due to occlusion is the most accurate description of the condition.