Q. A 22-year-old woman comes to the clinic due to increasing fatigue and difficulty concentration during the past month. She started a strict vegan diet 8 years ago. On clinical examination, she has glossitis and pale conjunctiva. On neurological examination, she has severe proprioceptive and vibratory loss, normal cold temperature sensation, brisk reflexes, normal motor power but significant ataxia that severely limits her mobility. You order iron studies. Using the accompanying table, what is the most likely diagnosis?