**Core Concept**
The radial nerve is a branch of the brachial plexus that innervates several muscles in the posterior arm and forearm, including the triceps brachii, anconeus, and extensor muscles of the forearm. Injury to the radial nerve can result in paralysis of these muscles.

**Why the Correct Answer is Right**
When the radial nerve is injured below the spiral groove, the muscles of the posterior arm are spared, but the extensor muscles of the forearm are affected. The spiral groove is a landmark that marks the transition from the posterior arm to the forearm. The triceps brachii, which is located in the posterior arm, is innervated by the radial nerve before it reaches the spiral groove. However, the extensor muscles of the forearm, including the extensor carpi radialis brevis, extensor carpi radialis longus, and extensor digitorum, are innervated by the radial nerve after it passes the spiral groove. Therefore, when the radial nerve is injured below the spiral groove, these forearm muscles are paralyzed.

**Why Each Wrong Option is Incorrect**
* **Option A:** The triceps brachii is located in the posterior arm and is innervated by the radial nerve before it reaches the spiral groove. Therefore, it is not affected by an injury to the radial nerve below the spiral groove.
* **Option B:** The brachialis muscle is innervated by the musculocutaneous nerve, not the radial nerve. Therefore, it is not affected by an injury to the radial nerve.
* **Option C:** The pronator teres muscle is innervated by the median nerve, not the radial nerve. Therefore, it is not affected by an injury to the radial nerve.

**Clinical Pearl / High-Yield Fact**
The radial nerve injury below the spiral groove results in a classic clinical presentation known as "wrist drop," characterized by weakness or paralysis of the extensor muscles of the forearm, leading to difficulty extending the wrist and fingers.

**Correct Answer:** C. Pronator teres is not the correct answer. The correct answer is likely the extensor muscles of the forearm.