Q. A 47 year-old man patient presents with a few-month course of progressive burning pain across the shoulders. Neurological exam reveals: - weakness of the upper extremities; - absent biceps reflexes with hyperreflexia of lower extremities; - pathologic (ie, positive) Babinski's; - absent pain/temperature sensation across the upper chest and limbs with preserved vibration/proprioception sensation. Which of the following is the most likely diagnosis?