The clinical problem links loss of shoulder abduction, elbow flexion and forearm supination to injury at Erb's point.
Structures involved at Erb's point
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The ventral rami of the fifth and sixth cervical segments join to form the upper trunk.
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The six structures listed at the injury site are the fifth and sixth cervical ventral rami, suprascapular nerve, nerve to subclavius, and anterior and posterior divisions of the upper trunk.
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The suprascapular nerve and nerve to subclavius arise from the upper trunk.
Motor deficits and posture
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The patient cannot abduct the shoulder, flex the elbow or supinate the forearm.
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The upper trunk divisions supply fibres to deltoid, brachialis, biceps brachii and supinator.
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The elbow is extended and the forearm is pronated in Erb's paralysis.
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The supplied answer key associates porter's tip or policeman's tip deformity with Erb's paralysis and identifies triceps brachii as the exception among the listed muscles weakened.
Additional anatomical distinctions
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The supplied answer key identifies the suprascapular nerve as the exception among the listed posterior cord branches.
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The medial root of the median nerve is the exception among the listed lateral cord branches.
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Teres minor is the exception among the listed muscles forming the posterior wall of the axilla.