The enzyme responsible for the digestion of lactose in the small intestine is lactase. Lactase breaks down lactose into glucose and galactose, which are absorbable monosaccharides. Deficiency of lactase leads to lactose intolerance, causing gastrointestinal symptoms due to undigested lactose fermentation by colonic bacteria. Other disaccharidases like maltase and sucrase act on different substrates, and amylase targets starch, not lactose. Therefore, lactase is the specific enzyme essential for lactose digestion in the small intestine.

Reference: Guyton and Hall, Textbook of Medical Physiology, 14th Edition.