You are answering a USMLE-style medical question. Question: A 63-year-old man presents to the physician with a complaint of back pain for the past 2 months. The pain is present throughout the day, and even at night when he lies down. He also complains of difficulty walking upstairs and says he recently started to wear adult diapers because he seems to have difficulty controlling his bowel movements. His vital signs are within normal limits. The neurological examination revealed bilateral lower limb weakness, diminished temperature sensation, and decreased vibratory sense. The rectal examination revealed a hard nodular mass and a weak rectal sphincter tone. Which of the following is the most likely cause of his symptoms? Options: A) Syringomyelia B) Spinal metastasis C) Herniated disc D) Spinal hematoma Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.