Q. A 60-year-old woman presents to your family practice clinic with a history of persistent fatigue and paresthesias in both arms and legs for the past 6 months. She looks pale. Neurological examination shows bilateral symmetric impaired position and vibration senses in the legs more than arms. Lab results are shown below. Further tests indicate low vitamin B12 (cobalamin) levels. Which of the following amino acids is most likely elevated in this patient?