Which muscle forms the medial border of the cubital fossa?

The muscle that forms the medial border of the cubital fossa is the pronator teres. This muscle originates from the medial epicondyle of the humerus and extends down to the mid-radius, functioning primarily to facilitate the pronation of the forearm. In the context of the cubital fossa, the boundaries are defined by specific muscles and structures, and the pronator teres plays a crucial role in demarcating the medial aspect of this anatomical space.

The brachioradialis primarily forms the lateral border of the cubital fossa and is involved in forearm flexion, while the triceps brachii is situated posteriorly in the arm and does not contribute to the borders of the fossa. The biceps brachii, particularly its tendon, does cross the cubital fossa but does not form a border; instead, its tendon lies in the center of the fossa. Thus, the pronator teres is clearly the muscle that defines the medial limit of the cubital fossa, making it the correct choice.