**Core Concept**
Pointing index finger is a classic sign seen in nerve injury, specifically involving the radial nerve. The radial nerve is responsible for the extension and abduction of the wrist and fingers, as well as the sensation of the back of the hand and the thumb.

**Why the Correct Answer is Right**
The correct answer is related to the radial nerve's role in finger extension. When the radial nerve is injured, the extensor muscles of the wrist and fingers are weakened, leading to an inability to extend the fingers. As a result, when attempting to point with the index finger, the finger remains flexed and the individual is unable to fully extend it. This is due to the impaired function of the extensor digitorum communis muscle, which is innervated by the radial nerve.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the ulnar nerve is responsible for the innervation of the intrinsic muscles of the hand, including the adductor pollicis, which is not related to the pointing index finger sign.
* **Option B:** This option is incorrect because the median nerve is responsible for the innervation of the flexor digitorum profundus muscle, which is involved in finger flexion, not extension.
* **Option C:** This option is incorrect because the musculocutaneous nerve is responsible for the innervation of the biceps brachii and brachialis muscles, which are involved in elbow flexion, not finger extension.

**Clinical Pearl / High-Yield Fact**
The pointing index finger sign is a classic clinical sign that can be used to diagnose radial nerve injury. It is essential to remember that this sign is related to the radial nerve's role in finger extension, making it a high-yield fact for medical students and clinicians.

**Correct Answer: D. Radial nerve injury.