A 65-year-old man has experienced worsening fatigue for the past 5 months. On physical examination, he is afebrile and has a pulse of 91/min, respirations of 18/min, and blood pressure of 105/60 mm Hg. There is no organomegaly. A stool sample is positive for occult blood. Laboratory findings include hemoglobin of 5.9 g/dL, hematocrit of 18.3%, MCV of 99 mm3, platelet count of 250,000/ mm3, and WBC count of 7800/ mm3. The reticulocyte concentration is 3.9%. No fibrin split products are detected, and direct and indirect Coombs test results are negative. A bone marrow biopsy specimen shows marked erythroid hyperplasia. Which of the following conditions best explains these findings?

The marked reticulocytosis and marrow hyperplasia indicate that the marrow is responding to a decrease in RBCs. The reticulocytes are larger RBCs that slightly increase the MCV. Anaplastic marrow is very hypocellular and unable to respond to anemia; it is associated with pancytopenia. The normal Coombs test results exclude autoimmune hemolytic anemia. Iron deficiency impairs the ability of the marrow to mount significant and sustained reticulocytosis. Iron deficiency anemia is typically microcytic and hypochromic but could be partially masked here by reticulocytosis, which would not be as marked if iron were not available, but his diet is supplying needed iron. Infiltrative disorders, such as metastases in the marrow, would impair the ability to mount reticulocytosis of this degree.