A day after a left-sided lumpectomy and axillary dissection, a 63-year-old woman is experiencing difficulty elevating her left arm. She cannot fully raise her upper arm from the side of her body. The median border and inferior angle of the left scapula become unusually prominent when she pushes against the wall with both hands. The innervation of which of the following muscles was most likely injured during the surgery?

This patient most likely sustained an injury to the long thoracic nerve (C5-C7), which innervates the serratus anterior muscle. This muscle is located on the lateral poion of the thorax, attaching to the external surface of the lateral pas of the 1st - 8th ribs proximally and the anterior surface of the medial border of the scapula distally. It is responsible for keeping the scapula applied to the thoracic wall, and protracts and rotates the scapula. Other muscles and bones rely on the scapula to be "fixed", as they use it as an anchor when producing movements of the humerus. Injury to the long thoracic nerve, which lies on the medial wall of the axilla, can occur during weight lifting, thoracic or axillary surgery, or from a stab wound. The clinical features of a long thoracic nerve injury include a "winged" scapula (the protrusion of the scapula as the patient presses against a wall) and an inability to abduct the arm. The deltoid and teres minor muscles are innervated by the axillary nerve (C5-C6), which winds around the humeral neck, and may be injured during the dislocation of the shoulder or a fracture to the neck of the humerus. Injuries to this nerve present with weakness of shoulder abduction, and atrophy of the shoulder. "Winging" of the scapula is not a prominent clinical feature of an axillary nerve injury. The latissimus dorsi muscle is innervated by the thoracodorsal nerve (C6-C8), and is a large, fan-shaped muscle that extends, adducts, and medially rotates the humerus, and helps raise the body toward the arms during climbing and performing chin-ups. Injury to the nerve that innervates this muscle can occur during surgical operations in the axilla, and makes the patient unable to perform a chin-up or go climbing. The patient's clinical findings in this case are inconsistent with an injury to the thoracodorsal nerve. The pectoralis major muscle, which is innervated by the medial and lateral pectoral nerves, is involved in the adduction and medial rotation of the arm. Injury to the nerves that innervate this muscle does not typically result in any disability, however the anterior axillary fold is absent on the affected side.