**Core Concept**
Erb's palsy is a type of brachial plexus injury that affects the upper limb, specifically involving the C5-C6 nerve roots. This injury leads to a characteristic deformity due to the paralysis of the muscles that control arm movement.

**Why the Correct Answer is Right**
The characteristic deformity of Erb's palsy is typically described as the "waiter's tip" deformity, where the affected arm hangs in adduction and medial rotation. This occurs because the paralysis of the supraspinatus and infraspinatus muscles (which control abduction and lateral rotation) is more pronounced than the paralysis of the subscapularis muscle (which controls medial rotation). As a result, the arm is pulled into a position of adduction and medial rotation, resembling a waiter's tip.

**Why Each Wrong Option is Incorrect**
**Option A:** Adduction and lateral rotation of arm - This is incorrect because the affected arm in Erb's palsy is typically in medial rotation, not lateral rotation. Lateral rotation is controlled by the supraspinatus and infraspinatus muscles, which are often paralyzed in Erb's palsy.
**Option C:** Abduction and lateral rotation of arm - This is incorrect because the affected arm in Erb's palsy is typically in adduction, not abduction. Abduction is controlled by the supraspinatus and deltoid muscles, which are often paralyzed in Erb's palsy.
**Option D:** Abduction and medial rotation of arm - This is incorrect because the affected arm in Erb's palsy is typically in adduction, not abduction.

**Clinical Pearl / High-Yield Fact**
The "waiter's tip" deformity is a classic sign of Erb's palsy, and it is often used as a clinical indicator of this condition. It is essential to recognize this deformity in order to diagnose and manage Erb's palsy effectively.

✓ Correct Answer: B. Adduction and medial rotation of arm