<s>[INST] context: Subclan steal syndrome refers to steno-occlusive disease of the proximal subclan aery with retrograde flow in the ipsilateral veebral aery and associated cerebral ischaemic symptoms.CausesClinical presentationIpsilateral upper limbNeurological, radiationCongenital Weak or absent pulseDecreased blood pressure (>20 mmHg)Arm claudication (rare due to collateral perfusion)DizzinessVeigoSyncopeAtaxiaVisual changesDysahriaWeaknessSensory disturbancesRadiographic featuresUltrasoundCT angiographyMR angiographyRetrograde flow in ipsilateral veebral aeryEarly changes prior to reversal of flow: decreased velocity, biphasic flow (in veebral aery)Changes can be augmented with arm exercise or inflation of BP cuff above systolic pressureProximal subclan aery usually can't be seen well enough to assessDistal subclan aery shows parvus-tardus waveform and monophasic waveformSubclan aery stenosis or occlusion easily identifiedDelayed enhancement of ipsilateral veebral aeryDirection of flow in veebral aery cannot be determinedOther intracranial or extracranial cerebral vascular lesions can be identifiedSubclan aery stenosis or occlusion easily identifiedDelayed enhancement of ipsilateral veebral aeryRetrograde direction of flow in ipsilateral veebral aeryOther intracranial or extracranial cerebral vascular lesions can also be identifiedTreatmentEndovascular: angioplasty & stent inseionSurgical: bypass surgery [/INST] question: Subclan steal syndrome is due to | options: A: Blockage of first pa of subclan aery B: Blockage of second pa of subclan aery C: Blockage of third pa of subclan aery D: Thrombosis of subclan vein | answer: B </s>