A 42-year-old man had surgery to remove a tumor from his right lung three years ago. He received postoperative radiation therapy of 5000 cGy. His primary care physician has referred him for assessment of a painful right arm with weakness.
During the examination at the clinic, the patient appears alert but apprehensive. The right palpebral fissure is slightly smaller than the left, but eye movements are normal. The right pupil is smaller than the left. Swallowing and speech are normal, as is facial expression. The small muscles of the right hand are atrophied and weak, with clawing affecting the right ring and little fingers. There is decreased sensitivity to pinprick on the ulnar border of the right hand and forearm.
What is the most probable underlying diagnosis?