Neurological testing of a patient reveals no cutaneous sensation on the tip of the index finger. Such a finding would indicate injury to some poion of which nerve?
**Core Concept**
The question is testing the student's knowledge of the dermatomal distribution of cutaneous sensation and the corresponding spinal nerve roots. In this case, the dermatome corresponding to the tip of the index finger is C6, which is supplied by the medial cord of the brachial plexus and the musculocutaneous nerve.
**Why the Correct Answer is Right**
The musculocutaneous nerve is a branch of the lateral cord of the brachial plexus that arises from the roots of C5, C6, and C7. It innervates the biceps brachii, brachialis, and coracobrachialis muscles. The nerve then divides into the lateral cutaneous nerve of the forearm, which supplies the skin of the lateral forearm, and the anterior cutaneous nerve of the forearm, which supplies the skin of the medial forearm. However, the musculocutaneous nerve does not directly innervate the skin of the hand. Instead, the skin of the hand is innervated by the median nerve, which is formed by the medial and lateral cords of the brachial plexus and the T1 root. However, the median nerve does not supply the tip of the index finger. The tip of the index finger is actually supplied by the digital nerve that arises from the median nerve but this nerve is not the correct answer as the question asks for the nerve responsible for the dermatome and not the digital nerve itself. The correct answer is related to the spinal nerve root C6 which is responsible for the dermatome of the tip of the index finger.
**Why Each Wrong Option is Incorrect**
* **Option A:** The radial nerve is responsible for the sensation of the back of the arm and the lateral aspect of the forearm, but not the tip of the index finger.
* **Option B:** The ulnar nerve is responsible for the sensation of the little finger and the medial aspect of the hand, but not the tip of the index finger.
* **Option D:** The axillary nerve is responsible for the sensation of the deltoid region and the shoulder, but not the tip of the index finger.
**Clinical Pearl / High-Yield Fact**
The dermatomes of the upper limb correspond to the spinal nerve roots as follows: C5 (lateral forearm), C6 (tip of the index finger), C7 (middle finger), C8 (little finger), and T1 (ulnar aspect of the hand). This knowledge is essential for diagnosing nerve injuries and compressive neuropathies.
**Correct Answer:** C.