Posterior column sensations in lower limbs are lost in :

Deficiency of vitamin B12 leads to megaloblastic anemia which is indistinguishable from folic acid deficiency. Deficiency also have manifestations related to loss of myelin-like subacute combined degeneration of spinal cord (symptoms of lesions of a posterior column like loss of vibration and proprioception, paraesthesias, depressed stretch reflexes and mental changes like poor memory and hallucinations etc.)