**Core Concept**
The cubital fossa is a triangular region on the anterior view of the elbow, bounded by several anatomical landmarks. Understanding the boundaries of the cubital fossa is crucial in clinical practice, particularly when performing injections, venipunctures, or assessing the integrity of surrounding structures.

**Why the Correct Answer is Right**
The lateral boundary of the cubital fossa is formed by the **biceps brachii tendon**. This tendon is a continuation of the muscle belly of the biceps brachii, which originates from the scapula. The tendon inserts into the radial tuberosity, playing a crucial role in elbow flexion and forearm supination. The biceps brachii tendon serves as a key landmark for various clinical procedures, including the administration of intramuscular injections.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect as the medial epicondyle of the humerus forms the medial boundary, not the lateral boundary, of the cubital fossa.
* **Option B:** The brachioradialis muscle, while present in the cubital fossa, does not form the lateral boundary. Instead, it is a superficial muscle that assists in forearm flexion.
* **Option D:** The lateral margin of the pronator teres muscle is not a significant landmark for the lateral boundary of the cubital fossa.

**Clinical Pearl / High-Yield Fact**
The cubital fossa is divided into two parts by an imaginary line connecting the two bony prominences of the elbow: the medial epicondyle and the lateral epicondyle. This division is important for clinical procedures, as it helps identify the location of the median nerve and the radial nerve.

**Correct Answer:** C. **Biceps brachii tendon**