**Core Concept:** The cubital fossa is a depression on the medial aspect of the forearm, located between the medial epicondyle of the humerus and the medial border of the ulna. The lateral border of the cubital fossa is formed by the muscles and structures that surround it.

**Why the Correct Answer is Right:** The lateral border of the cubital fossa is formed primarily by the two muscles that are located in the region, namely the biceps brachii and the brachialis muscles. These muscles are essential in human anatomy and physiology, particularly in the context of forearm movements and muscle strength.

**Why Each Wrong Option is Incorrect:**
A. **Option A:** The pronator teres muscle is incorrect because, while it is present in the cubital fossa, it contributes to the formation of the medial border, not the lateral border.
B. **Option B:** The radial artery is incorrect because it is a blood vessel and does not contribute to the formation of the lateral border of the cubital fossa.
C. **Option C:** The ulnar nerve is incorrect as it is a nerve rather than a muscle and does not form part of the lateral border of the fossa.
D. **Option D:** The brachialis muscle is incorrect as it forms part of the lateral border of the cubital fossa, but it is not the only muscle contributing to it.

**Clinical Pearl:** Understanding the anatomy of the cubital fossa and its muscles is crucial for diagnosing and treating conditions such as cubital tunnel syndrome, which is caused by compression of the median nerve as it passes through the cubital tunnel formed by the bony structure of the medial epicondyle and the posterior border of the flexor retinaculum.

**Correct Answer:** The correct answer is **D** - the brachialis muscle. The brachialis muscle contributes to the formation of the lateral border of the cubital fossa by joining with the biceps brachii to form the supinator muscle at the level of the elbow joint. The supinator muscle then splits into two heads: the pronator teres and the pronator quadratus muscles.

The cubital fossa is a crucial region to know for medical students and healthcare professionals as it is a key area for clinical examination during physical assessments and for understanding various conditions affecting the elbow and forearm.