**Core Concept**
The dermatomal distribution of the upper limb is primarily controlled by the cervical spinal nerves. The C5 and C6 nerve roots are responsible for the innervation of the lateral aspect of the arm and forearm, as well as the lateral and proximal parts of the hand.

**Why the Correct Answer is Right**
The C5 and C6 nerve roots are involved in the formation of the musculocutaneous nerve (C5-C7), which provides motor innervation to the biceps brachii, brachialis, and coracobrachialis muscles. The sensory innervation of the lateral arm and forearm is provided by the musculocutaneous nerve through its terminal branch, the lateral cutaneous nerve of the forearm. Additionally, the C5 and C6 nerve roots contribute to the formation of the radial nerve, which provides sensory innervation to the back of the arm and forearm. The loss of C5 and C6 nerve root function would result in impaired sensation over the lateral aspect of the arm and forearm.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the C5 and C6 nerve roots do not provide primary innervation to the medial aspect of the arm and forearm, which would be affected by a C7 or C8 nerve root injury.
* **Option B:** This option is incorrect because the C5 and C6 nerve roots do not provide primary innervation to the medial aspect of the hand, which would be affected by a C8 or T1 nerve root injury.
* **Option C:** This option is incorrect because the C5 and C6 nerve roots do not provide primary innervation to the ulnar aspect of the hand, which would be affected by a C8 or T1 nerve root injury.

**Clinical Pearl / High-Yield Fact**
The dermatomal distribution of the upper limb can be remembered using the mnemonic "C5-C6: lateral arm and forearm, C7: middle finger, C8-T1: ulnar aspect of the hand."

**Correct Answer: D. Lateral aspect of the arm and forearm.**