a 20-year-old woman with sickle cell anemia whose usual hemoglobin concentration is 8 g/dl(80 g/l) (normal 14-17 g/dl) develops fever, increased weakness and malaise. the hemoglobin concentration is 4 g/dl (40 g/l) the reticulocyte count is 0.1 % (normal 0.5 - 1.5 %) . the most likely explanation for her clinical picture is: