## **Core Concept**
The radial nerve is a major peripheral nerve that provides motor and sensory innervation to the arm. It originates from the posterior cord of the brachial plexus and runs down the back of the humerus, supplying various muscles and skin areas. The autonomous zone of sensory supply refers to the area innervated by a specific nerve, where sensation is exclusively provided by that nerve.

## **Why the Correct Answer is Right**
The correct answer, **dorsum of the hand on the radial side (first web space)**, is the autonomous zone of sensory supply by the radial nerve. This area, specifically the first web space, is exclusively innervated by the radial nerve, particularly through its terminal branch, the superficial radial nerve. This area does not receive innervation from other nerves, making it the autonomous zone.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While the radial nerve does supply the back of the arm and forearm, the area over the deltoid is primarily innervated by the axillary nerve, not making it the autonomous zone for the radial nerve.
- **Option B:** The radial side of the forearm is supplied by the radial nerve, but it is not considered the autonomous zone because it can also receive some innervation from other nerves.
- **Option C:** The first three and a half fingers on the dorsum are supplied by the radial and ulnar nerves, making this not exclusively autonomous for the radial nerve.

## **Clinical Pearl / High-Yield Fact**
A key clinical point to remember is that the superficial radial nerve, a branch of the radial nerve, provides sensation to the dorsum of the hand, specifically the first web space. This is crucial for assessing radial nerve function, especially in cases of suspected nerve injury or neuropathy.

## **Correct Answer:** D. First web space.