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Clinical Reasoning: A teenager with left arm weakness

A 16-year-old right-handed boy presented with left arm weakness. He had broken his right humerus in November 2015 and was in cast until January 2016. During that time, he noticed left arm weakness while carrying a heavy backpack. The weakness improved and he was able to play lacrosse during spring 2016. In summer 2016, he noticed substantial weakness of his left arm after rowing 7 km, and sought medical attention. He denied any pain or sensory symptoms, lower limb weakness, and bladder or bowel symptoms. There was no preceding illness or trauma. On neurologic examination in October 2016, there was atrophy of the deltoid, biceps, and periscapular muscles, as well as mild scapular winging. There was weakness of the following muscles (right/left; Medical Research Council [MRC] grades): deltoids (5/3), biceps (5/4), supraspinatus (5/4−), infraspinatus (5/4−). First dorsal interosseous, abductor digiti minimi, abductor pollicis brevis, finger extensors and flexors, wrist extension/flexion, brachioradialis, triceps, and rhomboids were 5/5. Tendon reflexes were 2+ throughout and sensory examination was normal.