In obstetric brachial plexus palsy involving C5–C6 (Erb's palsy), the classical posture of the limb and the mechanism responsible are:

• A Abducted shoulder, elbow flexed, claw hand — loss of C8–T1 intrinsic muscles
• B Wrist drop, finger extension loss — radial nerve injury at the spiral groove
• C Adducted, medially rotated, elbow extended, forearm pronated ('waiter's tip') — loss of deltoid, biceps, brachialis, supraspinatus, infraspinatus ✓
• D Claw hand with intrinsic minus deformity — isolated ulnar nerve injury at the elbow

Explanation

Erb's palsy (C5–C6) affects deltoid (abduction), supraspinatus/infraspinatus (external rotation), biceps/brachialis (elbow flexion/supination), causing the unopposed posture: adduction, internal rotation, elbow extension, forearm pronation — the 'waiter's tip' deformity. Klumpke's palsy (C8–T1) causes intrinsic weakness and claw hand. Radial nerve injuries produce wrist drop, not this pattern.

Reference: Maheshwari Essential Orthopaedics, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.