Final answer:
The most common cause of clay-colored stools in a patient with cholelithiasis is a gallstone lodged in the common bile duct. This happens when the bile duct is blocked, causing bilirubin to back up into the liver and result in jaundice.
Step-by-step explanation:
The nurse should realize that the most common cause of clay-colored stools in a patient with cholelithiasis is (d) a gallstone lodged in the common bile duct. When a gallstone blocks the bile duct, bile cannot be excreted properly. As a result, bilirubin, the bile pigment, backs up into the liver and is passed via the blood, causing jaundice. This jaundice can lead to clay-colored stools.