**Core Concept:** Axonal and/or myelin injury in peripheral nerves can result in specific motor and sensory deficits based on nerve anatomy and function.

**Why the Correct Answer is Right:** In this scenario, the person is suffering from a motor deficit, specifically the inability to extend fingers (flexion). The injury has affected the flexor digitorum superficialis (FDS) and flexor digitorum profundus (FDP) nerves. These are the nerves responsible for finger flexion. The wrist extensor muscles, including the extensor digitorum tendon, are supplied by the radial nerve, which is not involved in this injury, allowing them to function normally.

**Why Each Wrong Option is Incorrect:**

A. This option refers to the median nerve, which is responsible for finger flexion and not extensor function.
B. The same explanation applies to this option, as the ulnar nerve controls finger flexion and is not involved in extensor function.
C. This option is incorrect because the posterior interosseous nerve controls wrist extension, not finger extension.
D. The brachial plexus is a network of nerves, not a single nerve, so this option is too broad and does not address the specific motor deficit described.

**Clinical Pearl:** Understanding peripheral nerve anatomy is crucial for diagnosing and treating nerve injuries. In this case, proper identification of the involved nerves allows for targeted management and rehabilitation.

**Correct Answer:** D. Brachial plexus

**Explanation:** As mentioned previously, the injury is affecting the FDS and FDP nerves responsible for finger flexion. The brachial plexus is formed by the union of the cervical roots C5-C8 and T1, which includes the FDS and FDP nerves. Therefore, the correct answer is the brachial plexus. The other options are incorrect due to their involvement in finger flexion or wrist extension, not the extensor deficit described in the question.