**Core Concept**
The cubital fossa is a triangular area at the front of the elbow joint, bounded by the bicipital aponeurosis, the brachialis muscle, and the pronator teres muscle. It contains vital structures that are essential for elbow function, including the median nerve, the brachial artery, and the biceps tendon.

**Why the Correct Answer is Right**
The median nerve, brachial artery, and biceps tendon are the primary contents of the cubital fossa. The median nerve passes through the cubital fossa, providing sensory and motor innervation to the forearm and hand. The brachial artery, which is a continuation of the axillary artery, passes through the cubital fossa and divides into the radial and ulnar arteries. The biceps tendon, which is the tendon of the biceps brachii muscle, attaches to the radius bone and plays a crucial role in elbow flexion.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the ulnar nerve is not a primary content of the cubital fossa. Instead, it passes through the cubital tunnel, which is located posterior to the medial epicondyle of the humerus.
* **Option B:** This option is incorrect because the radial nerve is not a primary content of the cubital fossa. It passes through the radial tunnel, which is located posterior to the lateral epicondyle of the humerus.
* **Option C:** This option is incorrect because the axillary nerve is not a primary content of the cubital fossa. It passes through the quadrangular space, which is located posterior to the humerus.

**Clinical Pearl / High-Yield Fact**
The cubital fossa is a common site for injuries, such as fractures and dislocations, which can compromise the function of the median nerve and brachial artery. It is essential to be aware of the anatomy of the cubital fossa to avoid such complications.

**Correct Answer:** D.