A 19-year-old university student presents to her GP with a 2-month history of fatigue, palpitations on exertion and shortness of breath when climbing stairs. She is vegetarian and has heavy menstrual periods. Examination shows pallor and a flow murmur. Hb is 86 g/L, MCV 70 fL, ferritin 6 micrograms/L. What is the most appropriate initial management?