A 38 years old female patient presented with gradually worsening unsteady gait as well as numbness and tingling in hands and feet. At the time of admission neurological examination revealed slight weakness of distal muscles, bilaterally absent ankle jerk reflexes, bilaterally extensor plantar reflexes, and loss of position and vibration senses in all extremities. Brain MRI was unremarkable. T2 weighted MRI of the cervical spinal cord showed high signal lesions in the posterior columns extending to the upper thoracic spinal cord . This finding is caused due to deficiency of?