Q. A 65-year-old man with a known history of chronic kidney disease (CKD) presents to your family practice clinic complaining of persistent fatigue, pale skin, and difficulty concentrating. His temperature is 98.6°F (37°C), pulse is 86 beats per minute, blood pressure is 130/70 mmHg and respiratory rate is 14 per minute. Physical examination shows pallor and bilateral pitting pedal edema. His recent laboratory results reveal a decrease in his hemoglobin level compared to prior reports, with current hemoglobin measuring 8.5 g/dL. The peripheral blood smear shows a normocytic normochromic blood picture. There is no history of gastrointestinal bleeding or any known nutritional deficiencies. What is the most likely underlying cause for the decline in his hemoglobin levels?