Thoracic outlet syndrome

A 23-year-old man comes to his primary care physician to evaluate the sudden onset of numbness and tingling in the right upper extremity following a workout. The patient was performing sit-ups when he experienced a sharp pain in his neck, followed by numbness and tingling in his right hand. He does not have any symptoms while at rest. He reports several similar episodes in the past. He is a bodybuilder. Vital signs are within normal limits. The right lateral neck appears larger when compared to the left lateral neck. The patient has no spinal tenderness to palpation. Which of the following is the most likely underlying cause of this patient’s symptoms?