1.  Correct choice: B

GALLSTONE TYPES—Cholesterol stones develop in bile supersaturated with cholesterol. Of the two types of pigment stones, black stones form in sterile bile from excess bilirubin in the context of hemolytic anemia, cirrhosis, or sclerosing cholangitis, while brown stones form in infected bile. E. coli and parasites (Ascaris lumbricoides and Opisthorchis sinensis) are the most common infectious agents found in bile. The organisms produce phospholipases and glucuronidases, with formation of free fatty acids and unconjugated bilirubin, leading to formation of calcium salts. Disappearing stones are those that spontaneously dissolve or pass through the common bile duct or a cholecystoenteric fistula. Intramural stones have become embedded in the gallbladder wall.