Ans. is 'a' i.e., Lactulose * Lactulose is not digested or absorbed from the small intestine and it therefore acts as substrate for ferment ation and, hence as a colonic acidifier and an osmotic laxative.* Digestion of Carbohydrate* The most abundant carbohydrate in our food is starch. The other forms of carbohydrate which are consumed in substantial amounts are sucrose and lactose.* Starch is a polysaccharide composed entirely of glucose units linked by 1,4- alpha linkage and 1, 6 branching points. What it means that carbon atom -1 of a glucose unit is linked to carbon atom - 4 of the adjacent glucose unit, and that bond is of alpha - type. At the branching points, carbon atom-1 is linked to carbon atom-6 of the adjacent glucose unit.* Digestion of starch starts in the mouth itself by salivary amylase (ptylin). But the hydrolysis by salivary amylase cannot proceed much because the optimal pH for the reaction is 6-7 which is very different from the highly acidic environment of the stomach. The process is resumed by 'pancreatic amylase' in the small intestine.* Further digestion of starch derivatives (Disaccharides) occurs by the enzymes (oligosaccharidases) present in the brush border of small intestinal epithelial cells.* Maltase (a-glucosidase) breaks 1: 4 linkages in maltose and maltotriose and releases glucose (two molecules of glucose from maltose and three molecules of glucose from maltotriose).* Isomaltase (a-limit dextrinase) breaks 1:6a linkages of alpha-limit dextrin and releases glucose. Isomaltase (a-limit dextrinase) is the only enzyme that attacks 1: 6a linkage.* Sucrose is hydrolysed into fructose and glucose by sucrase (an enzyme present in brush border of intestinal epithelium). Lactose is hydrolysed into galactose and glucose by lactase (b-glucosidase). Trehalase hydrolyzes trehelose (a 1:1 a - linked dimer of glucose) into two glucose molecules. Trehalose is found in mushrooms.