A 35-year-old man falls off his bicycle and lands on his right arm. He presents to the emergency department with pain and swelling in his upper arm. On examination, there is tenderness and crepitus at the midshaft of the humerus. Neurologically, he has weakness in wrist and finger extension, and his wrist remains flexed at rest. He also has diminished sensation over the dorsum of his hand. His brachial and radial pulses are intact. Which of the following structures is most likely injured?
