What is the most likely diagnosis for a patient with a nontender mass in the anterior aspect of the arm after a sudden "pop"?

A nontender mass in the anterior aspect of the arm following a sudden "pop" strongly suggests a biceps tendon rupture. This type of injury is characterized by an audible pop, which often occurs during sudden or excessive exertion, such as lifting something heavy or performing a vigorous activity. The rupture leads to the retraction of the muscle, which can create a distinct bulge or mass that can be palpated in the upper arm, often referred to as a "Popeye" deformity due to the biceps muscle's appearance.

Additionally, the location of the mass is crucial for diagnosis. In cases of a biceps tendon rupture, the mass is typically located in the anterior region of the arm, which aligns perfectly with the clinical presentation described. Other potential conditions, while related to shoulder or arm injuries, do not typically present with this specific combination of symptoms. Understanding the mechanics of the biceps tendon, the manner in which it can rupture, and the subsequent physical findings helps to clarify why this is the most likely diagnosis in this scenario.