Protein-Energy Malnutrition (PEM):

• Kwashiorkor: protein deficiency (with caloric intake) → hepatic albumin synthesis fails → hypoalbuminaemia → reduced plasma oncotic pressure → oedema, ascites, fatty liver, skin lesions. Typical in children abruptly weaned to high-carb, low-protein diets.
• Marasmus: total calorie + protein starvation → body catabolises all fat + muscle → extreme emaciation, no oedema (albumin maintained until very late).

Bile Duct Obstruction (Cholestasis):

• No bile in duodenum → no fat emulsification → fat malabsorption → steatorrhoea (greasy, fatty stools)
• Fat-soluble vitamins (A, D, E, K) not absorbed: vitamin A → night blindness; vitamin D → rickets/osteomalacia; vitamin E → neuropathy; vitamin K → impaired clotting (elevated PT) → bleeding tendency
• Clinical signs: dark urine (bilirubin via kidneys), pale stools (no bilirubin in faeces), jaundice

Acute Pancreatitis:

• Premature activation of trypsinogen inside pancreas → trypsin activates all proenzymes inside pancreas → autodigestion
• Elastase activated → digests blood vessel walls → haemorrhage
• Lipase activated → digests peripancreatic fat → saponification (chalky white fat necrosis)
• Triggers: gallstones, alcohol abuse. Elevated serum amylase + lipase (diagnostic)

Peptic Ulcer Disease (H. pylori):

• H. pylori produces urease (neutralizes local acid for survival) + toxins (VacA, CagA) → disrupts mucus layer + increases gastrin → excess HCl + damaged protection → ulceration of stomach/duodenal wall
• Treatment: triple therapy (2 antibiotics + PPI)

Lactose Intolerance (Lactase Deficiency):

• No brush border lactase → undigested lactose → large intestine → bacterial fermentation → gas ($H_{2}$, $CO_{2}$, $CH_{4}$) + osmotic diarrhoea
• Diagnosis: hydrogen breath test. Treatment: lactase supplements, dairy avoidance.

Antibiotic-Associated Vitamin K Deficiency:

• Broad-spectrum antibiotics kill vitamin K-synthesizing bacteria in large intestine → vitamin K2 depletion → impaired clotting factor synthesis (II, VII, IX, X) → bleeding tendency, elevated INR

Oral Rehydration Therapy (ORS) — Applied GI Physiology:

• Diarrhoea depletes $Na^{+}$ and water. ORS contains $Na^{+}$ + glucose.
• SGLT co-transports $Na^{+}$ and glucose together → sodium absorption drives water back into body by osmosis
• Even in damaged gut, SGLT transporters on brush border remain functional

Crohn's Disease (Terminal Ileum):

• Disrupts bile salt reabsorption (enterohepatic circulation) → fat malabsorption + vitamin B12 deficiency (B12 absorption site = terminal ileum, via intrinsic factor) → megaloblastic anaemia