A 45-Year-Old Woman Comes to the Physician with Imbalance and Repeated

A 45-year-old woman comes to the physician with imbalance and repeated falls over the last 3 months.  She has fallen twice in the last week and reports that the imbalance is worse in the dark.  She also has numbness of her toes and weakness of both lower extremities.  The patient is unable to climb stairs without support.  She denies headache, diplopia, blurry vision, back pain, upper extremity symptoms, urinary incontinence, or trauma.
She had Roux-en-Y gastric bypass 3 years ago for morbid obesity.  Her medical problems include hypertension and hyperlipidemia.  Her current medications are ramipril, fenofibrate, nasal cyanocobalamin (B12) , calcium carbonate, iron sulfate, multivitamins, and vitamin D.  She began taking over-the-counter zinc supplements 4 months ago.  She does not use tobacco, alcohol, or illicit drugs.
Her blood pressure is 120/80 mm Hg without orthostatic changes, and pulse is 68/min.  Her BMI is 32 kg/m².  Neurological examination reveals mild bilateral weakness of the ankle extensors and flexors.  There is loss of proprioception and touch sensation in her feet up to her ankles.  The ankle reflexes are absent but the knee reflexes are brisk.  There is bilateral extensor plantar response.  She is unstable in Romberg position with her eyes closed.  The remainder of the physical examination is unremarkable.
Laboratory results are as follows:

Which of the following is the best next step in management of this patient?

A) Check serum methylmalonic acid and homocysteine levels
B) Check serum copper levels
C) Check RBC folate levels
D) Order MRI of the thoracic spine
E) Perform nerve conduction studies