✅ Model Answer:
• Diagnosis: Lactose malabsorption (primary adult-type hypolactasia).
• Evidence: Positive hydrogen breath test (>20 ppm rise), acidic stool (pH 5.5), reducing substances positive, normal pancreatic function (elastase) and normal sweat chloride, negative celiac serology.
• Next step: Trial of lactose-free diet. If symptoms persist, evaluate for secondary causes (celiac, Crohn, SIBO). Genetic testing for LCT gene if needed.

✅ Model Answer:
• Lactose: A disaccharide composed of glucose and galactose, found in milk and dairy products.
• Enzyme: Lactase (β-galactosidase) is located on the brush border of the small intestine.
• Types:
- Primary (adult-type) hypolactasia: Genetic decrease in lactase activity after weaning (most common).
- Congenital lactase deficiency: Rare, autosomal recessive (LCT gene).
- Secondary lactase deficiency: Acquired due to injury to the intestinal mucosa (e.g., gastroenteritis, celiac disease, Crohn disease, SIBO).
• Pathophysiology: Unabsorbed lactose in the colon is fermented by bacteria → production of short-chain fatty acids, hydrogen gas (H₂), and carbon dioxide → osmotic diarrhea, bloating, flatulence, abdominal pain, and acidic stool (pH <5.5).

✅ Model Answer:
• GI symptoms:
- Diarrhea: Watery, osmotic (stops with fasting).
- Abdominal bloating and distension.
- Flatulence.
- Abdominal pain/cramping.
- Nausea, vomiting.
- Foul-smelling, frothy stools.
• Timing: Symptoms usually occur 30 minutes to 2 hours after lactose ingestion.
• Other features:
- Failure to thrive (if chronic and severe).
- Perianal excoriation (acidic stools).
- May be asymptomatic (tolerance varies).
• Age of onset: Primary hypolactasia typically manifests after age 5-6 years; secondary can occur at any age.

✅ Model Answer:
• Hydrogen breath test: Gold standard.
- Procedure: Patient ingests a lactose load (1-2 g/kg, max 50 g). Exhaled H₂ is measured every 30 minutes for 3 hours.
- Positive: Rise in H₂ >20 ppm above baseline.
- Advantage: Non-invasive, sensitive.
• Stool tests:
- Stool pH: <5.5 (acidic) suggests carbohydrate malabsorption.
- Stool reducing substances: >0.5% (positive) indicates unabsorbed sugars.
• Serum glucose test: A flat glucose curve after lactose load (rise <20 mg/dL) – less sensitive.
• Genetic testing: LCT gene (C/T-13910 polymorphism) for primary hypolactasia.
• Rule out secondary causes: Anti-tTG IgA, fecal elastase, sweat chloride, small bowel biopsy (if indicated).

✅ Model Answer:
• Dietary modification:
- Lactose-free or low-lactose diet: Avoid milk, ice cream, soft cheeses, and processed foods containing lactose.
- Lactose-reduced products: Lactose-free milk, yogurt (contains lactase), hard cheeses (low lactose).
- Calcium and vitamin D supplementation: To prevent deficiencies if dairy is restricted.
• Lactase enzyme supplements:
- Oral lactase drops/tablets: Taken with dairy products (e.g., Lactaid).
- Effective for: Mild to moderate lactose intolerance.
• Secondary lactose intolerance:
- Treat the underlying cause: Celiac disease, Crohn disease, SIBO, etc.
- Temporary lactose restriction: Usually resolves once the mucosa heals.
• Education: Reading food labels, identifying hidden sources of lactose (e.g., bread, cereals, processed meats).

✅ Model Answer:
• Nutritional:
- Calcium deficiency: Increased risk of osteopenia, osteoporosis.
- Vitamin D deficiency.
- Failure to thrive (if severe chronic diarrhea).
- Growth retardation.
• GI:
- Chronic diarrhea → dehydration, electrolyte imbalance.
- Perianal excoriation (from acidic stools).
- Abdominal pain, bloating → poor quality of life.
• Psychosocial:
- Dietary restrictions → social anxiety, school absenteeism.
• Secondary complications: If untreated underlying cause (e.g., celiac disease) → osteoporosis, anemia, lymphoma.

✅ Model Answer:
• Prognosis:
- Excellent with dietary modification.
- Primary: Lifelong dietary management (may improve with age in some individuals).
- Secondary: Resolves with treatment of the underlying cause.
- Calcium status: Requires attention to maintain adequate intake.
- Growth: Normal if diet is well-balanced.
• Long-term follow-up:
- Monitor growth: Height, weight, BMI.
- Monitor calcium and vitamin D levels: Especially if dairy is restricted.
- Dietary counseling: Ensure adequate calcium intake from non-dairy sources (e.g., fortified plant milks, leafy greens, supplements).
- Re-evaluate secondary causes if symptoms persist despite dietary changes.

✅ Model Answer:
• Hydrogen breath test (HBT): Non-invasive test to diagnose carbohydrate malabsorption.
• Principle: Unabsorbed lactose is fermented by colonic bacteria → production of hydrogen gas → hydrogen is absorbed into the bloodstream and exhaled in breath.
• Procedure:
- Fasting baseline H₂ measured.
- Lactose load (1-2 g/kg, max 50 g) is given orally.
- Breath H₂ is measured at 30-minute intervals for 3 hours.
• Interpretation:
- Positive: Rise >20 ppm above baseline (definitive for lactose malabsorption).
- Negative: No rise (lactose is likely absorbed).
• Advantages:
- Non-invasive, well-tolerated, sensitive, and specific.
- Can distinguish between malabsorption and intolerance.
• Limitations:
- May be false negative if bacterial overgrowth is absent (hydrogen producers).
- Can be false positive if SIBO is present (due to early hydrogen rise).
- Requires cooperation (difficult in young children).