Which statement correctly describes the microbiota's conversion of primary bile acids into secondary bile acids?

• A. Cholic acid → deoxycholic acid; Chenodeoxycholic acid → lithocholic acid
• B. Cholic acid → lithocholic acid; Chenodeoxycholic acid → deoxycholic acid
• C. Cholic acid → ursodeoxycholic acid; Chenodeoxycholic acid → muricholic acid
• D. Primary bile acids are not converted by microbiota

Answer: (A)

Gut microbes perform 7α-dehydroxylation of primary bile acids to form secondary bile acids. Cholic acid is converted to deoxycholic acid, and chenodeoxycholic acid is converted to lithocholic acid. This is the classic pathway by which secondary bile acids are produced in the human gut, with deoxycholic and lithocholic acids being the major products. Some bacteria can further modify bile acids to small extents (for example, forming ursodeoxycholic acid in certain contexts), and mice have additional primary bile acids like muricholic acids, but the main microbial conversions described are CA to DCA and CDCA to LCA.