Muscle spared in ERB’s palsy?
**Core Concept**
ERB's palsy is a form of neonatal brachial plexus injury that affects the upper trunk of the brachial plexus, specifically the C5 and C6 roots. This results in weakness or paralysis of the muscles innervated by these roots, leading to characteristic clinical features such as shoulder weakness, elbow flexion, and forearm supination.
**Why the Correct Answer is Right**
In ERB's palsy, the muscle groups affected include the supraspinatus, infraspinatus, deltoid, biceps brachii, and brachialis. The muscle spared in ERB's palsy is the trapezius, which is innervated by the spinal accessory nerve (cranial nerve XI) and is not part of the brachial plexus.
**Why Each Wrong Option is Incorrect**
* **Option A:** The supraspinatus muscle is involved in ERB's palsy, as it is innervated by the suprascapular nerve, which arises from the C5 root.
* **Option B:** The deltoid muscle is also affected in ERB's palsy, as it is innervated by the axillary nerve, which arises from the C5 and C6 roots.
* **Option C:** The biceps brachii muscle is involved in ERB's palsy, as it is innervated by the musculocutaneous nerve, which arises from the C5 and C6 roots.
**Clinical Pearl / High-Yield Fact**
The key to diagnosing ERB's palsy is to look for the classic "waiter's tip" deformity of the arm, with the forearm in supination and the wrist in flexion, combined with weakness of shoulder abduction and external rotation.
**Correct Answer:** D. Trapezius.