**Core Concept**
The radial nerve runs through the radial groove of the humerus, innervating various muscles in the posterior arm. This question tests the student's knowledge of the specific muscles innervated by the radial nerve and their anatomical relationship to the radial groove.

**Why the Correct Answer is Right**
The muscles innervated by the radial nerve include the triceps brachii, anconeus, and the extensor muscles of the forearm (extensor carpi radialis brevis, extensor carpi radialis longus, extensor carpi ulnaris, extensor digitorum, extensor digitorum minimi, extensor indicis, and extensor pollicis longus and brevis). However, the radial nerve to the triceps brachii originates before it enters the radial groove. Therefore, when the radial nerve is injured in the radial groove, the triceps brachii is spared. This is because the triceps brachii is innervated by a branch that arises from the radial nerve before it reaches the radial groove.

**Why Each Wrong Option is Incorrect**
**Option A:** The extensor carpi radialis brevis is innervated by the radial nerve, and its function would be affected by a radial nerve injury in the radial groove.
**Option B:** The extensor carpi ulnaris is innervated by the posterior interosseous nerve, a branch of the radial nerve that arises after the nerve enters the radial groove. Therefore, it would be affected by a radial nerve injury in the radial groove.
**Option C:** The extensor digitorum is innervated by the posterior interosseous nerve, a branch of the radial nerve that arises after the nerve enters the radial groove. Therefore, it would be affected by a radial nerve injury in the radial groove.
**Option D:** The extensor indicis is innervated by the posterior interosseous nerve, a branch of the radial nerve that arises after the nerve enters the radial groove. Therefore, it would be affected by a radial nerve injury in the radial groove.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the radial nerve injury in the radial groove would spare the triceps brachii, but would affect the function of the extensor muscles of the forearm. This is because the triceps brachii is innervated by a branch that arises before the radial nerve enters the radial groove.

**Correct Answer:** . Triceps brachii