A 43-year-old man comes to the physician with pain in his left leg. He has a background of a left below-the-knee amputation 10 years ago. He reports that since the amputation he has always been “aware” of his left foot and had the feeling that it was still there. With time, it felt that his foot was moving proximally, and additionally, he developed pain and “burning” at the stump of his left leg. The pain does not change with position of his leg, topical treatments or medications. He is most likely suffering from which of the following syndromes?