**Core Concept**
The cubital fossa is a triangular depression on the anterior aspect of the elbow, bounded by the bony and ligamentous structures of the elbow joint. The medial boundary of the cubital fossa is formed by the medial epicondyle of the humerus.

**Why the Correct Answer is Right**
The medial epicondyle of the humerus serves as the attachment point for the pronator teres muscle and the ulnar collateral ligament, which provides medial stability to the elbow joint. The medial epicondyle is a bony projection on the distal end of the humerus, located on the medial (inner) aspect of the elbow. Its position and orientation form the medial boundary of the cubital fossa, separating it from the rest of the medial aspect of the elbow.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the medial epicondyle of the humerus is the correct landmark for the medial boundary of the cubital fossa.
* **Option B:** This option is incorrect because the medial brachial cutaneous nerve, which supplies sensation to the medial aspect of the arm, is not a bony structure and therefore cannot form the medial boundary of the cubital fossa.
* **Option C:** This option is incorrect because the olecranon process of the ulna, which forms the posterior aspect of the elbow joint, is not a part of the cubital fossa and therefore cannot form its medial boundary.
* **Option D:** This option is incorrect because the medial antebrachial cutaneous nerve, which supplies sensation to the medial aspect of the forearm, is not a bony structure and therefore cannot form the medial boundary of the cubital fossa.

**Clinical Pearl / High-Yield Fact**
Remember that the medial boundary of the cubital fossa is formed by the bony landmark of the medial epicondyle of the humerus, which serves as the attachment point for the pronator teres muscle and the ulnar collateral ligament.

**Correct Answer:** C. Olecranon process of the ulna