**Core Concept**
The cubital fossa is a triangular region in the anterior view of the elbow, bounded by several muscles and bone structures. Understanding the borders of the cubital fossa is crucial for identifying the optimal site for various medical procedures, such as venipuncture and arterial cannulation.

**Why the Correct Answer is Right**
The lateral border of the cubital fossa is formed by the **brachioradialis muscle**. This muscle is a forearm flexor, responsible for flexing the forearm and supinating the wrist. Anatomically, it arises from the lower end of the lateral supracondylar ridge of the humerus and inserts into the distal end of the radius. The brachioradialis muscle plays a significant role in the medial rotation of the forearm and is often involved in the formation of the cubital fossa.

**Why Each Wrong Option is Incorrect**
**Option A:** Pronator teres - This muscle forms the medial border of the cubital fossa, not the lateral border.
**Option B:** Flexor Carpi Radialis - This muscle is involved in wrist flexion and radial deviation, and is not a border of the cubital fossa.
**Option C:** Flexor Carpi Ulnaris - This muscle is involved in wrist flexion and ulnar deviation, and is not a border of the cubital fossa.

**Clinical Pearl / High-Yield Fact**
The cubital fossa is a clinically important region, as it contains the median nerve, the brachial artery, and the median vein. Identifying the borders of the cubital fossa is crucial for performing various medical procedures safely and effectively.

**✓ Correct Answer: A. Brachiradialis**