An 80-year-old woman has a three month history of progressive numbness and unsteadiness of her gait. On examination, there is a mild spastic paraparesis, with brisk knee reflexes, ankle reflexes are present with reinforcement, extensor plantars, sensory loss in the legs with a sensory level at T10, impaired joint position sense in the toes, and loss of vibration sense below the iliac crests. Investigations were as follows:- haemoglobin 12.0 g/dl MCV 99 fl What is the most likely diagnosis? A. anterior spinal artery occlusion B. dorsal meningioma C. multiple sclerosis D. subacute combined degeneration of the cord E. tabes dorsalis