A 22-year-old comes to the emergency department with light-headedness, diarrhea and fatigue during the past 6 months. She has a 2-year history of gluten-sensitive enteropathy. Her pulse is 100/min. Physical examination shows generalized pallor and glossitis. A peripheral blood smear shows macroovalocytes and hypersegmented neutrophils. Her hemoglobin concentration is decreased, serum homocysteine concentration is increased, and serum methylmalonic acid concentration is within the reference range. This patient most likely has a deficiency of which of the following vitamins? a) Folic acid b) Niacin c) Vitamin B1 (thiamine) d) Vitamin B6 (pyridoxine) e) Vitamin B12 (cobalamin)

A 22-year-old comes to the emergency department with light-headedness, diarrhea and fatigue during the past 6 months. She has a 2-year history of gluten-sensitive enteropathy. Her pulse is 100/min. Physical examination shows generalized pallor and glossitis. A peripheral blood smear shows macroovalocytes and hypersegmented neutrophils. Her hemoglobin concentration is decreased, serum homocysteine concentration is increased, and serum methylmalonic acid concentration is within the reference range. This patient most likely has a deficiency of which of the following vitamins? a) Folic acid b) Niacin c) Vitamin B1 (thiamine) d) Vitamin B6 (pyridoxine) e) Vitamin B12 (cobalamin)