Injury to which nerve would result in motor weakness in the forearm without sensory loss?

• A. Dorsal root ganglion
• B. Posterior interosseous nerve
• C. Medial antebrachial cutaneous nerve
• D. Radial nerve in the arm

Answer: (B)

The posterior interosseous nerve fits this scenario because it is a mainly motor branch of the radial nerve in the forearm that innervates the extensor muscles. It carries little to no sensory fibers in that region, so injury causes motor weakness of forearm extension without any loss of sensation. In contrast, a dorsal root ganglion injury would affect sensory function, a purely cutaneous nerve like the medial antebrachial cutaneous nerve would cause sensory loss, and a lesion high in the radial nerve in the arm would produce both motor and sensory deficits in its distribution.