**Core Concept**
The 'pointing index finger' sign is a clinical indicator of a specific nerve injury, related to the **radial nerve**. This nerve is responsible for the extension of the wrist and fingers. Injury to this nerve can lead to weakness or paralysis of the extensor muscles.

**Why the Correct Answer is Right**
The correct answer is related to the function of the **radial nerve**, which when injured, can lead to an inability to extend the wrist or fingers, resulting in a characteristic posture where the patient can only extend the index finger, due to the intact function of the **extensor indicis** muscle, which is innervated by the **posterior interosseous nerve**, a branch of the radial nerve.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it is not directly related to the specific clinical sign of 'pointing index finger'.
**Option B:** Incorrect as it does not correlate with the nerve responsible for the 'pointing index finger' sign.
**Option C:** Incorrect because, although related to nerve injuries, it does not specifically match the 'pointing index finger' sign.
**Option D:** Incorrect as it is not the nerve injury associated with this particular clinical presentation.

**Clinical Pearl / High-Yield Fact**
The 'pointing index finger' sign is a classic indicator of a **radial nerve injury**, particularly at the level of the forearm, where the nerve's continuity is compromised, affecting the extension of the wrist and fingers, except for the index finger, due to the specific innervation pattern.

**Correct Answer:** D. Radial nerve injury