A 28-year-old male motorcyclist sustains a traction injury to the right brachial plexus during a high-speed collision. On examination, he has weakness of wrist and finger flexion, loss of hand grip strength, and a claw hand deformity. Sensation is lost over the medial aspect of the forearm and hand. Which nerve root injury best explains this clinical picture, and what is the most reliable distinguishing feature from a complete upper trunk injury?