A middle-aged vegetarian man presents with progressive atrophy and weakness of hands and forearms. On examination he is found to have slight spasticity of legs, and generalized hyperreflexia. MRI study if the brain was obtained and increased signals were noted in thecoicospinal tracts on T2 weighted MRI (shown below). The most likely diagnosis is
Multiple sclerosis Amyotrophic lateral sclerosis Subacute combined degeneration Progressive spinal muscular atrophy