How does an infant with Erb palsy typically hold their affected arm?

An infant with Erb's palsy, which results from injury to the upper trunk of the brachial plexus, typically presents with the affected arm hanging by their side and rotated medially. This characteristic positioning occurs due to weakness or paralysis of the muscles responsible for shoulder abduction and external rotation, particularly the deltoid and rotator cuff muscles.

When these muscles are compromised, the arm cannot be lifted or moved normally, leading to a reliance on gravity to hold the arm down in a relaxed position by the side. The medial rotation is a result of the unopposed action of muscles that are still functioning, such as the internal rotators, while the inability to oppose or counteract these forces results in the arm being held in this specific posture.

The alternative choices each describe different positions that do not accurately reflect the typical posture of an infant with Erb's palsy. Understanding this characteristic position is vital for healthcare providers in diagnosing and managing the condition, as well as for planning appropriate interventions for rehabilitation.