A 64 year old man is evaluated for decreased exercise tolerance and dyspnea on exercise for 3 weeks. He underwent gastric bypass surgery 6 month ago and has lost 27 kg. on examination, temp 36.7 C, BP 137/78 mmHg, HR 105/min, and RR 13/min. he has pale conjunctivae but normal cardiopulmonary examination. His initial laboratory investigations show a hemoglobin level of 7.4 g/dl, a mean corpuscular volume of 104. Which of the following is the most appropriate next diagnostic test?