This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Upper limb, Subject name: Anatomy
A patient woke up from sleep with difficulty in extending fingers. He can make a grip and hold apen. Wrist extension was possible. No sensory disturbance was found. Injury could be at 
Select and explain. 
A. C8,T1, B. Posterior interosseous nerve, C. Lower pa of brachial plexus, D. Hand area in motor coex
B: Posterior interosseous nerve
Answer: B: Posterior interosseous nerve 
Explanation: Posterior interosseous nerve emerges from supinator on the back of the forearm .and lies between the superficial and deep muscles.At the lower border of the extensor pollicis brevis, it passes deep to the extensor pollicis longus .it then runs on the posterior surface of the interosseous membrane up to the wrist where it enlarges into pseudoganglion and ends by supplying the wrist and intercarpal joints. Reference : page no: 136 - BD chaurasia,upper limb & thorax