Candice Nguyen is a 21-year-old student at a prestigious engineering school. During the past 6 months, she experienced several bouts of severe abdominal bloating and cramps, followed by diarrhea. At first, she thought these episodes were caused by the stress of her demanding academic program. However, she noticed that the symptoms occurred approximately 1 hour after she drank milk or ate ice cream. On a visit home, Candice mentioned the symptoms to her mother, who exclaimed, Dont you know that your father and I have never been able to drink milk? Candice was examined by her primary care physician, who found her to be in excellent health. Because Candices symptoms were temporally related to ingestion of dairy products, the physician ordered a lactose-H₂ breath test, which confirmed that Candice has lactose intolerance. Her fecal osmolar gap was measured and was elevated. As a further confirmation of the diagnosis, Candice abstained from dairy products for 1 week and had no episodes of bloating, cramping, or diarrhea.