A 50-year-old woman presents with a 3-month history of fatigue, generalised aches, and low mood. Blood tests reveal: Hb 110 g/L (115-165), MCV 78 fL (80-100), Ferritin 10 µg/L (15-300). Her diet is varied. Menstrual history is normal. What is the most appropriate next step in management after initiating oral iron supplementation?