A 26-year-old woman is brought to the emergency department because of a 2-day history of worsening shortness of breath. She is otherwise healthy and takes no medications. She appears pale. Her blood pressure is 95/65 mmHg and pulse is 125/min. Pulse oximetry on room air shows an oxygen saturation of 95%. Jugular venous pressure is elevated. Crackles are heard at both lung bases. A soft systolic murmur is heard over the left sternal edge. The peripheries are warm. Her hemoglobin concentration is 3.7 g/dL. A blood smear shows microcytic anemia. Which of the following is the most appropriate next step in management?