Which bilirubin form has higher CNS affinity, increasing the risk of kernicterus?

• A. B1 high
• B. B2 high
• C. Both high
• D. B1 low

Answer: (A)

The key idea is that unconjugated (indirect) bilirubin is lipophilic and can cross the blood–brain barrier, while conjugated (direct) bilirubin is water-soluble and does not cross easily. When unconjugated bilirubin is present at high levels, or when the free (unbound) fraction increases because albumin binding is reduced, it can accumulate in brain tissue and cause kernicterus. In the given forms, the B1 form corresponds to unconjugated bilirubin with high CNS affinity, so it carries the greatest risk for kernicterus. The B2 form, being conjugated bilirubin, has much lower CNS affinity and is less likely to cause kernicterus. Thus, high B1 indicates higher CNS affinity and greater risk.