A 32-year-old man presents to the outpatient clinic with yellowing of his eyes and skin noticed over the past 3 days. On examination, you observe yellow discoloration of the sclera and skin. The patient asks when jaundice becomes clinically visible. The threshold marked **C** in the diagram represents the serum bilirubin level at which clinical jaundice becomes detectable. What is this critical bilirubin concentration?