A 12-year-old boy is brought to the emergency department by his mother due to left hand clumsiness. Several hours ago, he was climbing a tree but lost his balance and fell, grabbing a branch with his left hand on the way down. He landed safely on his feet with no head injury or loss of consciousness. On physical examination, the patient has difficulty performing fine finger movements with his left hand. Which of the following structures is most likely injured in this patient? Axillary nerve, Musculocutaneous nerve, Long thoracic nerve, Inferior trunk of the brachial plexus, or Radial nerve?