173. A 9-month-old boy is brought to the emergency department because of an upper respiratory tract infection and decreased appetite for 2 days and rapid breathing for 6 hours. He is at the 3rd percentile for length and weight. He appears acutely ill. Physical examination shows lethargy and dehydration. Laboratory studies show ketosis, metabolic acidosis, hypoglycemia, anemia, and methylmalonic aciduria. He improves following treatment with intravenous fluids and glucose. Direct sequencing of the patient's leukocyte DNA fails to show mutations in the methylmalonyl-CoA mutase gene. In addition to dietary protein limitation, administration of which of the following is most appropriate for this patient? A) Biotin B) Folic acid C) Niacin D) Vitamin B6 (pyridoxine) E) Vitamin B12 (cyanocobalamin)

173. A 9-month-old boy is brought to the emergency department because of an upper respiratory tract infection and decreased appetite for 2 days and rapid breathing for 6 hours. He is at the 3rd percentile for length and weight. He appears acutely ill. Physical examination shows lethargy and dehydration. Laboratory studies show ketosis, metabolic acidosis, hypoglycemia, anemia, and methylmalonic aciduria. He improves following treatment with intravenous fluids and glucose. Direct sequencing of the patient's leukocyte DNA fails to show mutations in the methylmalonyl-CoA mutase gene. In addition to dietary protein limitation, administration of which of the following is most appropriate for this patient? A) Biotin B) Folic acid C) Niacin D) Vitamin B6 (pyridoxine) E) Vitamin B12 (cyanocobalamin)