A patient lacks small-intestinal lactase activity. Predict what happens to ingested lactose and which symptoms are most likely to develop shortly after milk consumption.

• A. Lactose is absorbed intact across the small intestine via facilitated diffusion and causes systemic hypoglycemia because it bypasses digestion; therefore, the main symptom is fainting but no GI upset.
• B. Lactase deficiency causes immediate stomach acid neutralization leading to heartburn without gas or diarrhea because the lactose directly neutralizes gastric acid.
• C. Undigested lactose reaches the colon where bacteria ferment it, producing gas, osmotically active metabolites, and short-chain fatty acids, leading to bloating, cramping, and diarrhea.
• D. Lactose spontaneously polymerizes into glycogen in the stomach preventing absorption, which causes constipation and decreased stool frequency as the primary symptom.

Answer: (C)