You are answering a USMLE-style medical question. Question: A 24-year-old woman comes to the physician because of persistent fatigue for the past 4 months. She has no history of major medical illness. Her temperature is 36°C (96.8°F), pulse is 121/min, and blood pressure is 120/78 mm Hg. Physical examination shows pale conjunctivae. A peripheral blood smear is shown. Which of the following is the most likely cause of this patient's symptoms? Options: A) Hereditary spherocytosis B) Vitamin B12 deficiency C) Iron deficiency D) β-thalassemia minor Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.