What would cause pain & weakness in arm from elbow down?

Neuro/radiculopathy : Pain and weakness may be an indication that there could be compression and/or irritation of the nerves that innervate your arm. The two most likely culprits are compressive lesions in the neck (cervical radiculopathy) or in your arms (peripheral neuropathy). A thorough history, physical exam, and other diagnostic modalities such as an EMG and MRI can help pinpoint the diagnosis.