Yоur next pаtient is а 68-yeаr-оld female patient whо presents to clinic for a health maintenance exam. A CBC is obtained with the following results: Complete Blood Count Results Value Reference Range WBC 6.5 4-11 x103/µL RBC 4.25 4.0-5 x 106/ µL Hgb 10.6 12.0-16 g/dL HCT 30.4 36-46% MCV 72.0 80-100fl MCH 24.2 28-34 pg MCHC 31.6 32-36g/dL RDW 15.4 12-14.6% PLT 260 150-450103/µL While reviewing this patient's chart you note that her hemoglobin was 10.8 last year and ferrous sulfate was prescribed, although patient indicates she did not take it. No other work up was done at that time. She is asymptomatic. Given the patient's age and lack of symptoms, you are suspicious that the anemia is due to a chronic underlying problem. The most common cause for this presenting anemia is: