Which of the following findings is most helpful in differentiating Klumpke's palsy from Erb's palsy in a neonate?

Explanation

Klumpke's palsy is a lower brachial plexus injury (C8, T1) primarily affecting the intrinsic hand muscles and wrist/finger flexors, leading to a 'claw hand' deformity. Erb's palsy is an upper brachial plexus injury (C5, C6, sometimes C7) affecting the shoulder and elbow muscles, resulting in a 'waiter's tip' deformity. Therefore, involvement of the intrinsic hand muscles is a key differentiating feature. Diaphragmatic paralysis (phrenic nerve C3-C5) can occur in severe upper plexus injuries (Erb's). Absence of Moro reflex is common in both if severe. Sensory loss over the deltoid region (axillary nerve C5-C6) is characteristic of Erb's palsy.