Parents bring a 5-day-old infant to your office. The mother is O negative and was Coombs positive at delivery. The term child weighed 3055 g (6 lb, 1 oz) at birth and had measured baseline hemoglobin of 16 g/dL and a total serum bilirubin of 3 mg/dL. He passed a black tarlike stool within the first 24 hours of life. He was discharged at 30 hours of life with a stable axillary temperature of 36.5°C (97.7°F). Today the infant's weight is 3000 g, his axillary temperature is 35°C (95°F), and he is jaundiced to the chest. Parents report frequent yellow, seedy stool. You redraw labs and find his hemoglobin is now 14 g/dL, and his total serum bilirubin is 13 mg/dL. The change in which of the following parameters is of most concern?
a) Hemoglobin concentration
b) Total serum bilirubin level
c) Axillary temperature
d) Weight