A 75-year-old woman was admitted to the hospital with mild h…

A 75-yeаr-оld wоmаn wаs admitted tо the hospital with mild heart failure. She complained of a tingling sensation in her calves and feet, ankle swelling, and, upon admission the nurse noted that she appeared confused and pale. Her hematology results returned with: Hb = 89 g/L, Hct = 0.29, MCV = 110 fL, and MCH = 40 pg. All serum biochemistry values were normal. Which vitamin should be tested for deficiency?