A 25-year-old man is brought to the emergency department after sustaining a stab wound to the lower back. Examination shows right leg weakness in all muscle groups and loss of vibration and proprioception on the same side. There is loss of pain and temperature sensation but preserved motor strength in the left leg. He has no problems with bladder or bowel retention. Motor strength is preserved in the upper extremities. His presentation is most consistent with which one of the following spinal cord syndromes?