A middle aged man presents with progressive atrophy and weakness of hands and forearms. On examination he is found to have slight spasticity of the legs, generalized hyper-reflexia and increased signal in the cortico-spinal tracts on T2 weighted MRI. The most likely diagnosis is -
Multiple selerosis
Amyotrophic latetal sclerosis
Subacute combined degeneration
Progressive spinal muscular atrophy