A 22-year-old college student presents to the student health clinic with intermittent abdominal bloating, cramping, flatulence, and diarrhea for the past 6 months. Symptoms occur approximately 1–2 hours after consuming milkshakes, ice cream, and other dairy products. He reports complete resolution of symptoms between episodes. He denies weight loss, fever, hematochezia, nocturnal symptoms, or family history of inflammatory bowel disease. Past medical history is negative, and he does not take any medications. Vital signs are within normal limits, and physical examination is normal.
Which of the following is the most likely underlying mechanism?
The correct answer is:
B) Deficiency of brush-border lactase
This patient has classic lactose intolerance. The most important clue is the predictable association between symptoms and dairy consumption. His episodic bloating, abdominal cramping, flatulence, and diarrhea occurring shortly after ingestion of milk products are highly characteristic. Equally important are the findings that are absent: he has no weight loss, anemia, nocturnal symptoms, gastrointestinal bleeding, or persistent symptoms between episodes, making inflammatory or malabsorptive disorders less likely.
Lactase is a brush-border enzyme located on the epithelial surface of the small intestine. When lactase activity is reduced, lactose cannot be adequately broken down into glucose and galactose. The undigested lactose remains within the intestinal lumen, creating an osmotic gradient that draws water into the bowel and causes diarrhea. Colonic bacteria subsequently ferment the lactose, producing hydrogen gas and short-chain fatty acids that contribute to bloating, abdominal discomfort, and flatulence.
Primary lactase deficiency is one of the most common digestive disorders worldwide and often develops after childhood. Diagnosis is frequently clinical, although hydrogen breath testing may be used when the diagnosis is uncertain. Management consists of dietary lactose restriction, use of lactase enzyme supplements, or consumption of lactose-free dairy products.
Answer choice A: Autoimmune destruction of intestinal villi, is incorrect.
This describes celiac disease. Patients typically have more persistent symptoms and may develop iron deficiency anemia, weight loss, osteoporosis, or other manifestations of malabsorption.
Answer choice C: Exocrine pancreatic insufficiency, is incorrect.
Pancreatic insufficiency causes fat malabsorption, steatorrhea, weight loss, and nutritional deficiencies. Symptoms are not specifically triggered by dairy products.
Answer choice D: Gluten-induced intestinal inflammation, is incorrect.
This also describes celiac disease. Gluten-containing foods rather than dairy products trigger symptoms.
Answer choice E: Inflammatory destruction of the colon, is incorrect.
Inflammatory bowel disease (IBD) causes chronic inflammation, often with bleeding, weight loss, elevated inflammatory markers, or nocturnal symptoms. The episodic dairy-associated nature of this patient’s symptoms argues strongly against IBD.
Key Learning Point
Lactose intolerance results from lactase deficiency in the small-intestinal brush border, leading to osmotic diarrhea and bacterial fermentation of undigested lactose after dairy consumption.
