Nerve root for dermatome supplying thumb and index finger:
**Core Concept**
The dermatome map is crucial in clinical neurology, correlating specific skin areas with nerve roots. The **cervical spine** and its corresponding nerve roots supply the upper limb. Understanding dermatomal distribution helps in diagnosing nerve root lesions.
**Why the Correct Answer is Right**
The thumb and index finger are primarily supplied by the **C6 dermatome**. This is because the **C6 nerve root** originates from the cervical spine and its sensory distribution includes the lateral aspect of the forearm, thumb, and index finger. The **radial nerve**, which is often involved in injuries affecting this dermatome, is responsible for the motor supply but the sensory component for these digits is primarily from **C6**.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it does not correspond to the dermatomal supply of the thumb and index finger.
**Option B:** Incorrect as it relates to a different dermatome.
**Option D:** Incorrect because it is associated with a lower dermatome.
**Clinical Pearl / High-Yield Fact**
Remembering the dermatome map is key, with **C6** being responsible for the thumb and index finger, **C7** for the middle finger, **C8** for the fourth and fifth fingers. This knowledge aids in localizing nerve root lesions.
**Correct Answer:** C. C6