National Physical Therapy Examination (NPTE) Practice Exam

The site likely causing weakness of elbow flexion and shoulder abduction after a backpacking trip is the upper trunk of the brachial plexus. This is due to the anatomy and function of the nerves that originate from this region.

The upper trunk of the brachial plexus is formed by the union of the upper two roots (C5 and C6) and contributes to several important nerves, including the axillary and musculocutaneous nerves. Weakness in shoulder abduction occurs because the axillary nerve, which innervates the deltoid muscle responsible for this action, branches off from the upper trunk.

Elbow flexion weakness can arise from dysfunction in the musculocutaneous nerve, which also receives contributions from the upper trunk. Therefore, injury to the upper trunk would affect both the axillary and musculocutaneous nerves, leading to combined weaknesses in shoulder abduction and elbow flexion.

In contrast, the other nerves listed, such as the axillary nerve, musculocutaneous nerve, and long thoracic nerve, would not independently account for both weaknesses in these actions when their innervations are considered. The long thoracic nerve specifically affects shoulder girdle stability rather than directly causing elbow flex