✅ Model Answer:
• Diagnosis: Carbohydrate malabsorption (likely lactose intolerance or secondary lactase deficiency).
• Evidence: Stool pH <5.5, reducing substances positive, normal fecal elastase (excludes pancreatic insufficiency), normal sweat chloride (excludes CF).
• Next step: Lactose breath test or trial of lactose-free formula. Evaluate for underlying cause (post-enteritis, celiac, congenital sucrase-isomaltase deficiency).

✅ Model Answer:
• Definition: Inability to digest and absorb dietary carbohydrates (lactose, sucrose, maltose, starch).
• Causes:
- Primary: Congenital lactase deficiency (rare), sucrase-isomaltase deficiency, glucose-galactose malabsorption.
- Secondary: Post-infectious (viral gastroenteritis), celiac disease, Crohn disease, bacterial overgrowth, protein-energy malnutrition.
- Acquired: Adult-type hypolactasia (most common).
• Mechanism: Unabsorbed carbohydrates in the colon are fermented by bacteria → production of short-chain fatty acids and gases (H₂, CO₂, methane) → osmotic diarrhea, abdominal distension, flatulence, acidic stool (pH <5.5), and reducing substances in stool.

✅ Model Answer:
• Chronic watery diarrhea: Osmotic diarrhea (stops with fasting).
• Abdominal distention: Due to gas production from bacterial fermentation.
• Failure to thrive: Malabsorption of calories and nutrients.
• Perianal excoriation: Due to acidic stools.
• Flatulence, bloating, crampy abdominal pain.
• Lactose intolerance: Symptoms occur 30 minutes to 2 hours after lactose ingestion.
• Other features: Nausea, vomiting, irritability (in infants).

✅ Model Answer:
• Stool tests:
- Stool pH: <5.5 (acidic) suggests carbohydrate malabsorption.
- Stool reducing substances: Positive (>0.5%) indicates unabsorbed carbohydrates (glucose, galactose, lactose, fructose).
- Stool microscopy: Fat globules (if fat malabsorption co-exists).
• Breath tests:
- Lactose breath test: Rise in exhaled H₂ >20 ppm after lactose load (for lactose intolerance).
- Fructose breath test, sucrose breath test.
• Serum tests:
- Serum glucose: Flat curve after carbohydrate load.
- Anti-tTG IgA: To rule out celiac disease.
- Pancreatic function: Fecal elastase-1 (normal in isolated carbohydrate malabsorption).
• Genetic testing: For congenital disorders (LCT, SI, SGLT1).
• Small bowel biopsy: For secondary causes (celiac, Crohn, etc.).

✅ Model Answer:
• Dietary modification:
- Lactose intolerance: Lactose-free or low-lactose diet; use of lactase enzyme supplements.
- Sucrase-isomaltase deficiency: Sucrose-free diet, use of sacrosidase enzyme.
- Glucose-galactose malabsorption: Fructose-based formula, avoid glucose/galactose.
- Secondary malabsorption: Treat the underlying cause (celiac, Crohn, etc.).
• Nutritional support:
- Elemental formula: For severe cases (e.g., congenital disorders).
- Monitor growth: Calorie supplementation if needed.
- Vitamin and mineral supplementation: If deficiencies are present.
• Probiotics: May help in some cases (limited evidence).
• Education: Avoid offending sugars, read food labels carefully.

✅ Model Answer:
• Failure to thrive: Chronic diarrhea leads to malnutrition and poor weight gain.
• Dehydration and electrolyte disturbances: Osmotic diarrhea causes water and electrolyte loss.
• Perianal excoriation: Acidic stools cause skin breakdown.
• Diaper dermatitis.
• Nutritional deficiencies: Iron, zinc, calcium, and fat-soluble vitamins (if co-existing fat malabsorption).
• Growth retardation.
• Social and psychological impact: Dietary restrictions, school absenteeism.

✅ Model Answer:
• Prognosis:
- Excellent with dietary modification.
- Primary (congenital): Lifelong dietary restriction; good if diagnosed early.
- Secondary: Resolves with treatment of the underlying cause.
- Acquired (adult-type): Managed with dietary avoidance; no long-term morbidity.
- Growth and development: Normal if treated early.
• Long-term follow-up:
- Monitor growth: Regular height and weight measurements.
- Monitor nutritional status: Iron, zinc, calcium, vitamin D.
- Dietary counseling: Ensure balanced diet, especially calcium and vitamin D intake (if dairy is avoided).
- Reassess for secondary causes: If symptoms persist despite dietary changes.

✅ Model Answer:
• Carbohydrate malabsorption:
- Stool pH: <5.5 (acidic).
- Reducing substances: Positive.
- Stool appearance: Watery, frothy.
- Fecal elastase: Normal.
- Dietary treatment: Remove offending carbohydrates (lactose-free, sucrose-free).
- Complications: Dehydration, perianal excoriation.
• Fat malabsorption:
- Stool pH: Normal (neutral).
- Reducing substances: Negative.
- Stool appearance: Bulky, foul-smelling, greasy, floats (steatorrhea).
- Fecal elastase: Low (<100 μg/g) in pancreatic insufficiency.
- Dietary treatment: Pancreatic enzyme replacement (PERT), MCT oil, fat-soluble vitamins.
- Complications: Fat-soluble vitamin deficiencies (A, D, E, K).