A 42 year-old woman presents with difficulty walking and numbness/tingling. She underwent gastric bypass surgery 10 years ago and has not been compliant with diet instructions since then. She lost 200...

A 42 year-old woman presents with difficulty walking and numbness/tingling. She underwent gastric bypass surgery 10 years ago and has not been compliant with diet instructions since then. She lost 200 pounds with her surgery. Her examination shows sensory loss to vibration and proprioception in a stocking glove distribution with hyperreflexia in her bilateral upper extremities and patella (with crossed adductors). Her ankle jerks are absent. Strength examination shows mild weakness in bilateral dorsiflexion, plantar flexion, toe flexion and toe extension. Gait has both steppage and some spasticity observed. Cerebellar examination is normal. Some of her laboratory studies are pending. However, her hemoglobin is slightly low at 10.4 grams/deciliter with an MCV of 118. Her methylmalonic acid is quite elevated at 1000. MRI of the cervical spine shows non-enhancing hyperintensities in the posterior and lateral spinal cord. Which deficiency is  likely etiology of the patients’ symptoms, examination, available laboratory test results and imaging?