Final answer:
The fluctuating bilirubin levels in the serial determinations are most likely due to sample exposure to light, as bilirubin is light-sensitive which affects the results.
Step-by-step explanation:
The serial bilirubin determinations show fluctuations over the course of three days, and the best explanation for the results is likely sample exposure to light. Bilirubin is sensitive to light, which can degrade it and lead to lower results as seen on Day 2. The reading went back up on Day 3 after the sample was assayed shortly after collection, reducing the chance for light-induced degradation. Possible causes like sample hemolysis, sample left in warm location, or reagent deterioration usually result in consistent increases or decreases in bilirubin levels, not the fluctuation observed here. Given that bilirubin levels can have clinical significance, such as risk of kernicterus in infants or determining the severity of jaundice in patients, it's important to handle samples properly to avoid false readings.