<s>[INST] context: A properly performed interscalene block universally blocks the ipsilateral phrenic nerve. Interscalene Block: Side Effects Diaphragmatic hemiparesis from ipsilateral phrenic nerve block (100% incidence, 25% reduction in pulmonary function) Pneumothorax Permanent neurologic injury Blockade of vagus, recurrent laryngeal, and cervical sympathetic nerves (Horner's syndrome) Epidural or subarachnoid injection Veebral aery injection Severe hypotension and bradycardia (Bezold-Jarisch reflex) The interscalene block is indicated for procedures on the shoulder and proximal humerus as well as the lateral two thirds of the clavicle. The interscalene block can also be utilized for surgery of the arm or forearm; however, the higher incidence of incomplete blockade of the inferior trunk with this technique may provide inadequate analgesia in the ulnar distribution. The patient's positioning and comfo, the surgeon's preferences, and the duration of surgery may necessitate coadministration of a general anesthetic. An interscalene catheter may be inseed for prolonged postoperative analgesia [/INST] question: The most common complication of interscalene block:- | options: A: Phrenic block B: Pneumothorax C: Hypotension D: Horner syndrome | answer: A </s>