**Question:** Which of the following is not a branch of the posterior cord of brachial plexus?

A. C5
B. C6
C. C7
D. C8

**Correct Answer:** D. C8

**Core Concept:** Brachial Plexus

The brachial plexus is a network of nerves formed by the union of spinal nerves C5-C8 and T1. It is a crucial component of the peripheral nervous system, responsible for controlling the movements and sensory functions of the upper limb. The posterior cord of the brachial plexus consists of spinal nerves C5, C6, and C7, while the anterior cord consists of nerves from T1.

**Why the Correct Answer is Right:**
C8 is not part of the posterior cord of the brachial plexus because it belongs to the anterior cord, which is formed by T1 spinal nerve. The posterior cord is formed by spinal nerves C5, C6, and C7, which are responsible for supplying motor and sensory innervation to the posterior regions of the upper limb.

**Why Each Wrong Option is Incorrect:**
A. C5 is part of the posterior cord, specifically supplying the intrinsic muscles of the hand.
B. C6 is also part of the posterior cord, contributing to motor innervation of the intrinsic muscles of the hand.
C. C7 is part of the posterior cord, contributing to motor and sensory innervation of the posterior regions of the upper limb.

**Why Each Wrong Option is Incorrect:**
C8 is part of the anterior cord (T1), supplying the sensory innervation to the anterior and lateral regions of the upper limb and the entire arm.

**Clinical Pearl:**
Understanding the division of the brachial plexus into posterior and anterior cords is crucial for clinical practice, as it helps in diagnosing and treating conditions involving the upper limb, such as assessing injury, compression, or involvement of the plexus. For example, if a patient complains of weakness in the posterior regions of the upper limb, a physician would suspect an injury to the posterior cord (C5-C8), while a complaint of sensory loss in the anterior regions would point towards an injury to the anterior cord (T1).