## **Core Concept**
The question tests knowledge of the sensory innervation of the hand, specifically the dorsum of the first webspace. This area is crucial for clinical examination, especially in diagnosing nerve injuries. The sensory nerves of the hand are derived from various branches of the brachial plexus.

## **Why the Correct Answer is Right**
The correct answer, **D. Dorsal branch of radial nerve (or superficial radial nerve)**, is right because the dorsum of the first webspace is primarily supplied by the superficial radial nerve, a branch of the radial nerve. This nerve provides sensory innervation to the back of the hand, particularly the area over the first webspace, which is the region between the thumb and the index finger.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because the **median nerve** primarily supplies the palmar surface of the thumb, index, middle finger, and the radial half of the ring finger, as well as the thenar muscles. It does not supply the dorsum of the hand.
- **Option B:** This option is incorrect because the **musculocutaneous nerve** primarily supplies the flexor muscles of the arm (biceps brachii, brachialis, and coracobrachialis) and the skin of the lateral forearm via the lateral antebrachial cutaneous nerve. It does not supply the dorsum of the hand.
- **Option C:** This option might seem plausible because the **ulnar nerve** does supply some areas of the hand, including the little finger and the ulnar half of the ring finger, but it does not supply the dorsum of the first webspace.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that the superficial radial nerve can be easily damaged, leading to loss of sensation over the dorsum of the thumb and the first webspace. This can occur due to wrist injuries, lacerations, or iatrogenic causes like arterial cannulation. Clinicians should be aware of this when assessing hand injuries.

## **Correct Answer:** D. Dorsal branch of radial nerve.