**Core Concept:** Radial nerve is a mixed nerve that arises from the cervical spinal cord, supplying motor functions to the muscles of the forearm and sensory functions to the posterior aspect of the arm and the dorsum of the hand. A fracture in the shaft of humerus can potentially compress or stretch the radial nerve, leading to symptoms such as paresthesia, weakness, or anesthesia in the distribution of the radial nerve.

**Why the Correct Answer is Right:** To exclude radial nerve involvement, we need to assess the patient's symptoms and signs. The correct answer, **A**, involves examining the patient's radial nerve function. Assessing radial nerve function includes:

1. **Option A:** Motor function of the extensor muscles of the forearm (e.g., extensor carpi radialis brevis, extensor carpi ulnaris, extensor digitorum communis, and extensor indicis proprius). Assessing wrist extension, finger extension, and thumb extension will also be helpful.
2. **Option B:** Sensory function to the posterior aspect of the arm and dorsum of the hand. Palpate the radial sensory distribution and observe the patient's response to pinprick or light touch.
3. **Option C:** Motor function of the intrinsic muscles of the hand (e.g., abductor pollicis brevis, opponens pollicis, flexor pollicis brevis, and flexor digitorum profundus). Assessing thumb and finger flexion will also be helpful.
4. **Option D:** Motor function of the extensor muscles of the forearm (e.g., extensor carpi radialis longus, extensor digitorum, and extensor pollicis longus). Assessing wrist extension, finger extension, and thumb extension will also be helpful.

**Why Each Wrong Option is Incorrect:**

1. **Option C:** Assessing the motor function of the intrinsic hand muscles does not directly test the radial nerve function. These muscles are supplied by the ulnar nerve.
2. **Option D:** Assessing the motor function of extensor muscles of the forearm does not directly test the radial nerve function. These muscles are supplied by the ulnar nerve.

**Clinical Pearl:**

When assessing the radial nerve function, a clinical pearl to keep in mind is the **"3-2-1"** rule:

1. **3-2-1 Rule:** Assess radial nerve function in three areas: radial pulse, radial cutaneous nerve, and radial sensory nerve.
2. **2-1 Rule:** Assess two upper limb movements (wrist extension and finger extension) and one lower limb movement (thumb extension) to assess radial nerve function.
3. **1 Rule:** Assess radial sensory function (insensitivity to light touch and pinprick sensation).

By following the "3-2-1 Rule," you can comprehensively assess radial nerve function, ensuring that you cover the radial nerve's motor function, sensory function, and peripheral vascular supply. This helps to identify any deficits in radial nerve function, which is crucial when assessing a patient with a suspected radial nerve injury.