Which cervical nerve supplies the thumb dermatome

**Core Concept**
The dermatomes are areas of skin that are supplied by nerves that arise from specific spinal nerve roots. The cervical nerves (C1-C8) supply the dermatomes on the neck and upper limb. Understanding the dermatomal distribution is crucial for localizing neurological deficits and diagnosing conditions such as nerve root compression.

**Why the Correct Answer is Right**
The thumb dermatome is supplied by the C7 nerve root. This is because the C7 nerve root gives rise to the median antibrachial cutaneous nerve, which provides sensory innervation to the palmar surface of the thumb. The skin of the thumb is also innervated by the palmar cutaneous branch of the median nerve, which originates from the C7-C8 nerve roots. However, the primary sensory innervation to the thumb comes from the median antibrachial cutaneous nerve, which is a branch of the C7 nerve root.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is likely incorrect because the dermatome of the index finger is supplied by the C8 nerve root, not the thumb.
* **Option B:** This option is incorrect because the dermatomes of the radial side of the forearm and hand are supplied by the C5-C6 nerve roots, not the thumb.
* **Option C:** This option is incorrect because the dermatomes of the ulnar side of the forearm and hand are supplied by the C8-T1 nerve roots, not the thumb.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the dermatomal distribution is not always symmetrical, and there can be variations in the innervation of specific areas. This is particularly important in clinical practice when localizing neurological deficits or diagnosing conditions such as nerve root compression.

**Correct Answer: C. C7 nerve root.**