**Core Concept**
The cubital fossa is a triangular area on the anterior view of the elbow, bounded by several muscles and ligaments. It is an important anatomical landmark for various medical procedures and clinical assessments.

**Why the Correct Answer is Right**
The brachioradialis muscle is the primary boundary of the cubital fossa. It originates from the lower end of the lateral supracondylar ridge of the humerus and inserts into the distal end of the radius. As it wraps around the elbow, it forms the lateral boundary of the cubital fossa. This muscle plays a crucial role in forearm supination, particularly in the mid-pronated position.

**Why Each Wrong Option is Incorrect**
**Option A:** The pronator teres muscle does not form a boundary of the cubital fossa. It is located on the medial aspect of the forearm and is involved in forearm pronation.
**Option B:** The extensor carpi radialis brevis muscle is not a boundary of the cubital fossa. It is one of the extensor muscles of the wrist and forearm, but it does not contribute to the cubital fossa's boundaries.
**Option C:** The flexor carpi radialis muscle is not a boundary of the cubital fossa. It is involved in wrist flexion and radial deviation, but it does not form part of the cubital fossa's boundaries.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the brachioradialis muscle is a key landmark for the cubital fossa and is involved in various clinical assessments, such as blood pressure measurements and intravenous injections.

**Correct Answer: C. Brachioradialis.**