Regarding the conversion of bilirubin to biliverdin, which statements are true?

• A. Statements 1, 2, & 4 only.
• B. Statements 1 and 2 only.
• C. Statements 1, 2, & 3 only.
• D. Statements 1 and 3 only.

Answer: (A)

The key idea is the direction of bilirubin metabolism. In the body, heme is first converted to biliverdin by heme oxygenase, then biliverdin is reduced to bilirubin by biliverdin reductase. Bilirubin is subsequently conjugated for excretion. Because of this sequence, the physiologically standard reaction is biliverdin to bilirubin, not the reverse. There isn’t a common, normal pathway in humans that converts bilirubin back to biliverdin; any statement implying an active bilirubin-to-biliverdin conversion in typical physiology would be unsupported. That’s why the choice that aligns with this known pathway—the statements that reflect the biliverdin→bilirubin direction and the lack of a routine bilirubin→biliverdin reaction—is the best. The statement suggesting a normal bilirubin to biliverdin conversion contradicts this established route, making it the one that is not true.