Subclavian steal phenomenon is due to:

Ref: Harrison's 18th edition & WikipediaExplanation:If the subclavian artery is occluded proximal to the origin of the vertebral artery, there is a reversal in the direction of blood flow in the ipsilateral vertebral artery.Exercise of the ipsilateral arm may increase demand on vertebral flow, producing posterior circulation TIAs or "subclavian steal."The subclavian artery is divided for descriptive purposes into three parts by the anterior scalene muscle.Part 1 lies medial, part 2 lies posterior, and part 3 lies lateral to the anterior scalene muscle.BRANCHCOURSEPart 1VertebralInternal thoracicThyrocervial trunkAscends through C6 to C1 transverse foramina and enters foramen magnum Descend parasternaliy to anastomose with superior epigastric artery Gives rise to inferior thyroid, transverse cervical, and suprascapular arteriesPart 2Costocervial trunkGives rise to deep cervical and supreme intercostal arteriesPart 3Dorsa! scapularIs inconstant; may also arise from transverse cervical arteryHence Subclavian steal phenomenon is due to block in the first part of the subclavian artery.