Clinical Conditions and Digestive Physiology Connections

1. Lactose Intolerance

• Mechanism: Lactase deficiency (brush border) → undigested lactose → large intestine → bacterial fermentation → gas ($H_{2}$, $CO_{2}$, $CH_{4}$) + short-chain fatty acids + osmotic diarrhoea
• Diagnosis: Hydrogen breath test (exhaled $H_{2}$ elevated after lactose load)
• Treatment: Lactase enzyme supplements; dairy-free diet

2. Peptic Ulcer Disease (H. pylori)

• Mechanism: H. pylori → urease produces ammonia (survives stomach acid) → bacterial toxins (VacA, CagA) damage mucus layer + increase gastrin → excess HCl + disrupted mucus protection → pepsin and HCl attack stomach/duodenal wall → ulceration
• Treatment: Triple therapy (2 antibiotics + PPI)

3. Cholestasis (Bile Duct Obstruction)

• Mechanism: Bile cannot reach duodenum → no fat emulsification → fat malabsorption → steatorrhoea (greasy, fatty stools) + fat-soluble vitamin deficiency (A → night blindness; D → rickets/osteomalacia; E → neuropathy; K → bleeding disorders, elevated PT)
• Clinical clue: Dark urine (bilirubin excreted via kidneys) + pale stools (no bilirubin in bile/faeces)

4. Acute Pancreatitis

• Mechanism: Premature trypsinogen activation inside pancreas → trypsin activates all proenzymes inside the pancreas → autodigestion → elastase digests blood vessel walls (haemorrhage) + lipase digests peripancreatic fat (saponification/fat necrosis) → severe abdominal pain, elevated serum amylase + lipase
• Triggers: Gallstones (block pancreatic duct), alcohol, hyperlipidaemia

5. Crohn's Disease (Terminal Ileum)

• Mechanism: Inflammation of terminal ileum → disrupted bile salt reabsorption (enterohepatic circulation) → bile salt depletion → fat malabsorption; also disrupted vitamin B12 absorption (intrinsic factor-B12 complex absorbed only in terminal ileum) → B12 deficiency → megaloblastic anaemia

6. Kwashiorkor

• Mechanism: Protein deficiency → low albumin → low oncotic pressure → oedema + ascites + fatty liver
• Clinical sign: Oedema + distended belly in a child with some caloric intake (often high-carb, low-protein diet post-weaning)

7. Marasmus

• Mechanism: Total calorie + protein deficiency → fat + muscle wasting → extreme emaciation, no oedema
• Clinical sign: "Skin and bones" appearance, significantly underweight, preserved intellect initially

8. Hiatal Hernia / GERD

• Mechanism: Stomach herniates through diaphragm's oesophageal hiatus → lower oesophageal sphincter incompetence → acid reflux into oesophagus (no protective mucus there) → heartburn (burning chest pain), worse lying down (gravity aids reflux)

9. Cholera

• Mechanism: Vibrio cholerae toxin → activates adenylyl cyclase → cAMP accumulation → CFTR $Cl^{-}$ channels open → $Cl^{-}$ secreted into lumen → osmotic water loss → "rice-water" diarrhoea (up to 20 L/day)
• Treatment: ORS (uses SGLT co-transport — Na + glucose drive water back in)

10. Jaundice Types