1. Multiple hypoesthetic, mildly erythematous large plaques with elevated margins - this points away from pure tuberculoid (which has very few, well-defined lesions) and away from lepromatous (which has many small, poorly defined, symmetric lesions)
2. Bilateral enlarged peripheral nerves (ulnar + lateral popliteal) - nerve enlargement is prominent in the tuberculoid spectrum
3. Hypoesthesia - indicates significant immune-mediated nerve damage, typical of the tuberculoid end of the spectrum

• Few to several plaques (more than pure TT, but not numerous)
• Large plaques with raised, well-defined outer edges and sloping inner edges
• Marked hypoesthesia
• Peripheral nerve thickening (classically ulnar and lateral popliteal nerves)
• Paucibacillary on slit-skin smear

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Leprosy (Hansen's Disease) - Complete Guide

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PART 1: SIMPLE EXPLANATION (Beginner Level)

What is leprosy?

The One Big Concept: It's a SPECTRUM

STRONG IMMUNITY ←————————————————————→ WEAK IMMUNITY
  
      TT ——— BT ——— BB ——— BL ——— LL
  
   Few lesions,            Many lesions,
   few bacteria,           many bacteria,
   lots of nerve damage    less nerve damage early

• Left side (TT): Your immune system fights hard. It kills most bacteria. But in doing so, it destroys the nerves too. Few skin lesions, lots of nerve damage.
• Right side (LL): Your immune system barely responds. Bacteria multiply everywhere. Many skin lesions all over, but nerve damage comes later.
• Middle (Borderline types): Mix of both features.

Simple Memory Trick

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PART 2: DETAILED GUIDE (Final Year MBBS Level)

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The Organism

• Cannot be cultured in vitro (grown in labs)
• Grows best at 27-33°C (prefers cool body areas: skin, peripheral nerves, testes, nasal mucosa, ear lobes)
• Its virulence is partly due to PGL-1 (phenolic glycolipid-1), which helps it invade host cells
• Primarily infects Schwann cells and dermal macrophages
• A newer species, M. lepromatosis (discovered 2008), causes diffuse lepromatous leprosy mainly in Mexico/Central America

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Transmission

• Primarily via respiratory secretions (nasal droplets) from untreated multibacillary patients
• Rare zoonotic transmission from armadillos
• Prolonged close contact required; the disease is NOT highly contagious
• Incubation period: 2-12 years (average ~5 years)

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Immunology - The Key to Understanding the Spectrum

• HLA-DR2 and HLA-DR3 → tuberculoid form
• HLA-DQ1 → lepromatous form

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Classifications of Leprosy

1. Ridley-Jopling Classification (Most important for exams)

2. WHO Classification (Used in field/control programs)

3. Indian Classification (Used in India)

• Indeterminate, Tuberculoid, Borderline, Lepromatous, Pure Neuritic (no skin lesions - unique to Indian classification)

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Cardinal Features of Leprosy (Diagnose if ANY ONE present)

1. Hypopigmented/erythematous skin patch with loss of sensation
2. Thickened peripheral nerve trunk
3. AFB positive on slit-skin smear or biopsy

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Each Type in Detail

1. Indeterminate Leprosy (IL)

• Earliest stage - may be the only manifestation
• 1-2 ill-defined hypopigmented or faintly erythematous macules, 1-5 cm
• Occurs on limbs, buttocks, face
• Mild to moderate loss of touch/thermal sensation
• No nerve thickening
• Either self-heals or progresses to any determinate type depending on immunity
• Biopsy shows only lymphocytic infiltration (no granuloma yet)

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2. Tuberculoid Leprosy (TT)

• Single or very few (up to 3) well-defined lesions
• Raised, erythematous/copper-colored plaque with clearly defined outer edge sloping inward to a hypopigmented, depressed center (central healing)
• Can be of any size; surface is dry, hairless, anhidrotic, and completely anesthetic
• Complete loss of fine touch and temperature sensation over the lesion

• One peripheral nerve near the lesion may be thickened
• Asymmetric nerve involvement
• Nerve thickening with sensory loss and possible motor deficit

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3. Borderline Tuberculoid (BT) Leprosy

• 3 to 9+ lesions, asymmetrically distributed
• Plaques with edges sloping outward (opposite to TT)
• Smaller "satellite" lesions around the main lesion
• Variable loss of sensation (less than TT)
• Several peripheral nerves enlarged asymmetrically (ulnar, lateral popliteal commonly)
• Highly prone to Type 1 (reversal) reactions
• Bacteriologic Index (BI): 0 to 1+
• Lepromin test: Positive (weakly)
• If untreated, may downgrade toward BB or BL

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4. Mid-Borderline (BB) Leprosy

• Most immunologically unstable form - can upgrade or downgrade
• Multiple lesions of varying sizes and shapes
• Characteristic "Swiss cheese" (punched-out) lesions - central clearing with a sharp inner edge but sloping outer edge
• Lesions may look like annular plaques with a "punched-out" pale center
• Sensation impaired but not totally lost
• Nerves involved but less severely than BT
• BI: 2+ to 3+
• Lepromin test: Negative or weakly positive
• Most likely to undergo lepra reactions

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5. Borderline Lepromatous (BL) Leprosy

• Many lesions (more numerous and smaller than BB)
• Less-defined margins, more symmetric distribution than BT/BB
• Macules, papules, plaques, nodules
• Reduced sensation but not absent
• Nerve involvement: multiple nerves, more symmetric than BT
• BI: 3+ to 5+
• Lepromin test: Negative
• Can undergo Type 2 reactions (ENL - Erythema Nodosum Leprosum)

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6. Lepromatous Leprosy (LL)

• Many (dozens to hundreds) of lesions, symmetric distribution
• Early: poorly defined hypopigmented macules
• Late: papules, nodules, and plaques - especially on face, ears, wrists, elbows, knees
• Leonine facies = thickened, corrugated skin of face due to nodule coalescence
• Madarosis = loss of lateral eyebrows (pathognomonic)
• Saddle-nose deformity (nasal collapse from septal destruction)
• Ear lobes thickened and infiltrated
• Sensation relatively preserved early

• Symmetric peripheral nerve involvement
• Nerves invaded with bacteria but minimal inflammation
• Commonly affected: ulnar, median, lateral popliteal, posterior tibial, radial cutaneous, facial, greater auricular
• "Glove and stocking" sensory loss pattern

• Orchitis → testicular atrophy → sterility and gynaecomastia
• Eye involvement: iridocyclitis, corneal anesthesia, lagophthalmos
• Laryngeal involvement → hoarseness
• Lymphadenopathy
• Hepatosplenomegaly
• In advanced disease: bacilli in blood and sputum

• "Globi" = clumps of AFB in macrophages
• Virchow cells (lepra cells) = lipid-laden macrophages packed with AFB

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Histology at a Glance

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Nerves Commonly Affected (Must Know)

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Leprosy Reactions (Acute Episodes)

Type 1 Reaction (Reversal Reaction)

• Who: BT, BB, BL patients (borderline types only)
• Mechanism: Sudden increase in CMI (delayed hypersensitivity) - upgrade reaction
• Features: Existing lesions become acutely inflamed (red, hot, edematous, tender); new lesions; acute nerve palsies (very important!)
• Treatment: Prednisolone 40-60 mg/day, tapered over months

Type 2 Reaction - Erythema Nodosum Leprosum (ENL)

• Who: BL and LL patients only (multibacillary end)
• Mechanism: Immune complex deposition (Type III hypersensitivity) - NOT a change in CMI
• Features: Painful, tender, erythematous nodules on skin; fever, malaise; can have uveitis, orchitis, neuritis, arthritis, lymphadenopathy
• Treatment: Thalidomide (drug of choice), or clofazimine, or prednisolone

Lucio's Phenomenon

• Only in diffuse lepromatous leprosy (M. lepromatosis)
• Necrotizing vasculitis → skin necrosis and ulceration

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Diagnosis

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WHO MDT Treatment (Current Regimen)

Paucibacillary (PB) - 6 months

Multibacillary (MB) - 12 months

• Rifampicin: hepatotoxicity, orange urine
• Dapsone: hemolytic anemia (especially in G6PD deficiency), methemoglobinemia, dapsone hypersensitivity syndrome
• Clofazimine: skin pigmentation (orange-brown discoloration), ichthyosis, GI symptoms

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High-Yield Points for Exams

1. Most common nerve affected in leprosy overall: Ulnar nerve
2. Most common cause of foot drop in leprosy: Lateral popliteal (common peroneal) nerve
3. Pathognomonic skin finding: Madarosis (loss of lateral eyebrow)
4. Only nerve visible AND palpable: Greater auricular nerve
5. Lepromin test = test of immunity, NOT a diagnostic test
6. Most unstable type: BB (mid-borderline)
7. Type 1 reaction only in borderline; Type 2 (ENL) only in BL and LL
8. Drug of choice for ENL: Thalidomide (contraindicated in pregnancy - teratogenic)
9. BCG vaccination gives partial protection (~50%) against leprosy
10. Deformity of nose in LL = Saddle nose deformity (not "button nose")
11. "Leonine facies" = LL leprosy
12. Most common cause of blindness in leprosy = Lagophthalmos (facial nerve palsy → corneal exposure)
13. Pure neuritic leprosy (no skin lesions) - recognized in Indian classification only
14. Armadillos are the only non-human animal reservoir

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Sources

• Harrison's Principles of Internal Medicine, 22nd Edition, 2025 - Chapter 184
• Robbins, Cotran & Kumar - Pathologic Basis of Disease - Chapter 8
• Park's Textbook of Preventive and Social Medicine - Chapter on Leprosy

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PART 1: SIMPLE ORIENTATION (The Big Picture)

What is the GI System?

1. Take in food (ingestion)
2. Break it down (digestion - mechanical + chemical)
3. Absorb nutrients into the bloodstream
4. Expel waste (defecation)

The GI Tract at a Glance

Mouth → Pharynx → Esophagus → Stomach → Small Intestine (Duodenum → Jejunum → Ileum)
→ Large Intestine (Cecum → Ascending → Transverse → Descending → Sigmoid Colon)
→ Rectum → Anal Canal → Anus

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PART 2: ANATOMY - LAYER BY LAYER

General Wall Structure (Same Throughout the GI Tract)

Key clinical point: Auerbach's plexus (between circular and longitudinal muscle layers) controls motility. Loss of these ganglia = Hirschsprung's disease (aganglionic megacolon). - Yamada's Textbook of Gastroenterology, 7th edition

• Over 100 million neurons in the gut wall
• Functions largely independently of the CNS
• Two main plexuses: Meissner's (secretion/absorption) and Auerbach's (motility)
• Can coordinate entire peristaltic reflexes without input from the brain

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PART 3: SEGMENT-BY-SEGMENT ANATOMY AND PHYSIOLOGY

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1. MOUTH (Oral Cavity)

• Teeth: incisors (cutting), molars (grinding)
• Tongue: mixes food, contains taste buds (CN VII, IX, X)
• Salivary glands: parotid (largest, Stensen's duct → opposite upper 2nd molar), submandibular (Wharton's duct → floor of mouth), sublingual

• Jaw muscles innervated by CN V (trigeminal) motor branch
• Chewing controlled by reticular areas in brain stem, hypothalamus, amygdala
• Incisors generate ~55 lbs force; molars ~200 lbs
• Chewing reflex: bolus → jaw inhibition → stretch → rebound contraction (automatic rhythmic cycle)
• Function: increases surface area for enzyme access; especially important for plant foods with cellulose membranes

• Volume: ~1000-1500 mL/day
• Contents:
  • Ptyalin (salivary amylase): begins starch → maltose hydrolysis (α-1,4 bonds); continues 1-2 hours in stomach
  • Mucus: lubrication
  • IgA: antibacterial defense
  • Lysozyme: antibacterial
• Controlled by parasympathetic (CN VII, IX) stimulation → copious watery saliva; sympathetic → thick mucous saliva
• pH ~6.0-7.0

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2. PHARYNX AND ESOPHAGUS

• Length: ~25 cm; begins at C6 (cricopharyngeus) → ends at T10 (gastroesophageal junction, GEJ)
• Three anatomic narrowings (important for impaction/carcinoma):
  1. At cricopharyngeus (C6) - 15 cm from incisor
  2. At aortic arch/left main bronchus (T4) - 25 cm from incisor
  3. At GEJ/diaphragmatic hiatus (T10) - 40 cm from incisor
• Upper 1/3: striated muscle; Lower 2/3: smooth muscle; Middle: mixed
• Upper Esophageal Sphincter (UES): cricopharyngeus muscle (striated, normally CLOSED, ~60-80 mmHg)
• Lower Esophageal Sphincter (LES): smooth muscle, 3-4 cm above diaphragm (normally CLOSED, ~10-25 mmHg)

1. Soft palate elevates → seals nasopharynx
2. Palatopharyngeal folds approximate medially → form selective slit
3. Vocal cords approximate; larynx pulled up + forward → epiglottis tips back over larynx (airway protected)
4. UES relaxes → bolus enters esophagus

• Swallowing center: medulla oblongata
• Nerves involved: CN V, VII, IX, X, XII

• Primary peristalsis: initiated by swallowing reflex; progressive wave from UES to LES
• Secondary peristalsis: initiated by esophageal distension (even without swallowing); clears remaining bolus
• LES relaxes just before peristaltic wave arrives → bolus enters stomach
• LES is maintained closed by: intrinsic muscle tone, abdominal pressure, angle of His (cardiac angle), diaphragmatic crura

Clinical relevance: LES incompetence → GERD (gastroesophageal reflux disease). Failure of LES relaxation → Achalasia (absent peristalsis + incomplete LES relaxation). Loss of LES tone in pregnancy (progesterone) → heartburn in 80% of pregnancies.

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3. STOMACH

• J-shaped; volume ranges from ~30 mL (neonate) to 1.5-2 L (adult)
• Located at T10-L1; esophagogastric junction lies left of T10, gastroduodenal junction at L1
• Rotation during embryology: stomach rotates 90° around its longitudinal axis → explains why the left vagus innervates the anterior wall and right vagus innervates the posterior wall

• Greater curvature (left, inferior) - site of gastric vessels (gastroepiploic)
• Lesser curvature (right, superior) - "incisura angularis" is an important landmark; site of gastric ulcer

• Anterior: liver (right), anterior abdominal wall (left)
• Posterior: lesser sac, pancreas, transverse colon, spleen, left kidney, left adrenal (the "stomach bed")

• Right gastric + Left gastric (lesser curvature)
• Right gastroepiploic + Left gastroepiploic (greater curvature)
• Short gastric arteries (fundus, from splenic)
• All ultimately from celiac axis

• Follows blood supply to celiac nodes
• Clinical significance: gastric cancer spreads via lymphatics to Virchow's node (left supraclavicular), Troisier's sign

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Gastric Acid Secretion - In Detail

• Driven by H+/K+-ATPase (proton pump) on the canalicular membrane
• H+ pumped OUT into lumen; K+ pumped IN; Cl- follows H+ via channels
• Results in ~160 mmol/L HCl, pH ~0.8 (3 million times more acidic than arterial blood)
• "Alkaline tide": as H+ is secreted, HCO3- enters blood → gastric venous blood more alkaline than arterial

        GASTRIN (G cells)
              ↓
        ECL CELLS → HISTAMINE
              ↓
ACETYLCHOLINE (vagus) → PARIETAL CELL → HCl

Clinical relevance: Proton pump inhibitors (PPIs) - e.g., omeprazole - block the final common pathway (H+/K+-ATPase). H2 blockers - e.g., ranitidine - block histamine receptor. Vagotomy reduces acid by removing ACh stimulus - historically used for peptic ulcers.

• Acid itself (pH <3): inhibits G cells (feedback)
• Secretin (from S cells of duodenum): when duodenal pH <4.5 → inhibits gastrin and HCl
• CCK: inhibits gastric emptying
• GIP (Glucose-dependent Insulinotropic Peptide): from K cells → inhibits acid
• Somatostatin (D cells): paracrine inhibition

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Gastric Motility

• As food enters, fundus relaxes (vagally mediated, via VIP and NO) to accommodate 1.5-2L with minimal pressure rise
• Absent in post-vagotomy patients → early satiety

• Antrum acts as a pump (pyloric pump)
• Peristaltic waves originate from a pacemaker (gastric pacemaker) at mid-corpus, rate ~3/min
• Antrum contracts → chyme propelled toward pylorus → pylorus closes → chyme rebounds back = retropulsion and mixing

• Liquids: emptied rapidly (half-time ~20 minutes)
• Solids: require grinding into particles <1 mm before pylorus allows passage; half-time ~2-4 hours
• Fats empty most slowly

1. Duodenal distension
2. Duodenal acidity (pH <3.5-4)
3. Hyper/hypoosmolarity of chyme
4. Fat (fatty acids) → secretin + CCK → inhibit pyloric pump
5. Protein breakdown products

Clinical relevance: Gastroparesis (delayed gastric emptying) is common in diabetes (autonomic neuropathy) → early satiety, nausea, vomiting; treated with metoclopramide. Dumping syndrome post-gastrectomy: too-rapid gastric emptying → hyperosmolar load enters jejunum → massive fluid shift → early dizziness, late hypoglycemia.

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4. SMALL INTESTINE

1. 1st part (D1/duodenal bulb): 5 cm; intraperitoneal; posterior relations: gastroduodenal artery, bile duct, portal vein; site of >90% duodenal ulcers
2. 2nd part (D2/descending): 7-10 cm; retroperitoneal; Ampulla of Vater on posteromedial wall (bile + pancreatic duct entry); minor papilla 2 cm proximal
3. 3rd part (D3/horizontal): crosses vertebral column, IVC, aorta; crossed anteriorly by superior mesenteric vessels (SMA/SMV)
4. 4th part (D4/ascending): ends at duodenojejunal flexure (ligament of Treitz) = marker for upper vs. lower GI bleeding

Clinical relevance: SMA syndrome = 3rd part of duodenum compressed between aorta (posterior) and SMA (anterior) → vomiting after meals; occurs in severe weight loss.

1. Plicae circulares (Folds of Kerckring): permanent circular folds → 3x surface area
2. Villi: finger-like projections 0.5-1.5 mm tall → further 10x surface area
3. Microvilli (brush border): 1000 microvilli per cell, 1 μm long → another 20x

Total absorptive area = ~250 m² (size of a tennis court!) - Guyton & Hall, Medical Physiology

• Single layer of columnar epithelium (enterocytes) with tight junctions
• Central lacteal: lymphatic capillary that absorbs fat (chylomicrons)
• Blood capillaries: absorb water-soluble nutrients → portal vein → liver (first-pass metabolism)
• Goblet cells (mucus)
• Enteroendocrine cells

• Site of stem cells → cell renewal every 3-5 days
• Contain Paneth cells (antimicrobial peptides - defensins, lysozyme)

• Brunner's glands (in submucosa of duodenum only): secrete alkaline HCO3-rich mucus → neutralize gastric acid entering duodenum; Brunner's glands are stimulated by secretin and CCK
• Peyer's patches (in submucosa of ileum): lymphoid aggregates for immune surveillance; M cells overlying them sample luminal antigens

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5. LARGE INTESTINE

Cecum → Ascending colon → Hepatic flexure → Transverse colon → Splenic flexure 
→ Descending colon → Sigmoid colon → Rectum → Anal canal → Anus

• Teniae coli: 3 longitudinal bands of outer longitudinal muscle (instead of continuous coat) → cause colon to gather into haustra (sacculations)
• Haustra: pouches between teniae coli
• Appendices epiploicae: fat-filled peritoneal tags
• Wider caliber, no villi

• Cecum: blind pouch at ileocecal junction; intraperitoneal
• Ileocecal valve: prevents backflow from colon; opens transiently with each peristaltic wave
• Appendix: arises from posteromedial cecum, 2-3 cm below ileocecal valve; McBurney's point = 1/3 of the way from ASIS to umbilicus; all layers of gut present; contains lymphoid tissue (immune function); base projection of all 3 teniae coli = landmark to find appendix

• Rectum: 12-15 cm; follows sacral curve; 3 lateral curves (Houston's valves)
• Anorectal junction (at level of levator ani = puborectalis sling)
• Anal canal: 4 cm; has dentate (pectinate) line at midpoint
• Above dentate line: columnar epithelium, internal hemorrhoids; innervation visceral (autonomic) - painless
• Below dentate line: squamous epithelium, external hemorrhoids; innervation somatic (pudendal nerve) - painful
• Internal anal sphincter (IAS): smooth muscle, involuntary, tonically contracted (sympathetic)
• External anal sphincter (EAS): striated muscle, voluntary (pudendal nerve/somatic)

Clinical relevance: Hemorrhoids above dentate = internal (painless, bleed on defecation); below dentate = external (painful, thrombose). Fistula in ano - Park's classification based on relation to sphincters. Hirschsprung's disease - aganglionic segment always includes internal sphincter → absence of recto-anal inhibitory reflex (RAIR).

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PART 4: GI PHYSIOLOGY - DIGESTION AND ABSORPTION

GI Hormones - The Master Controllers

Mnemonic for CCK functions: "CCK - Contracts gallbladder, Curtails gastric emptying, Kicks out pancreatic enzymes"

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Pancreatic Secretion

1. Aqueous component (from duct cells): NaHCO3-rich fluid → neutralizes gastric acid in duodenum; stimulated by secretin
2. Enzymatic component (from acinar cells): digestive enzymes; stimulated by CCK + vagus

Clinical relevance: Acute pancreatitis = premature activation of trypsinogen inside acinar cells → autodigestion. Trypsin then activates all other zymogens. Enterokinase (enteropeptidase) deficiency → failure to activate trypsinogen → malabsorption + failure to thrive in infants.

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Digestion of the Three Macronutrients

Carbohydrate Digestion

• Glucose + Galactose: SGLT-1 (Na+/glucose cotransporter, secondary active transport) on apical membrane → basolateral exit via GLUT-2
• Fructose: GLUT-5 (facilitated diffusion) on apical → GLUT-2 basolateral

Clinical relevance: Lactase deficiency (most common enzyme deficiency worldwide, especially in East Asians and Africans) → undigested lactose enters colon → fermented by bacteria → gas, bloating, osmotic diarrhea. Glucose-galactose malabsorption = mutation in SGLT-1 → severe neonatal diarrhea.

Protein Digestion

• Amino acids: active transport with Na+ cotransporters (multiple, specific for groups)
• Di- and tripeptides: PepT1 (H+-coupled cotransporter) on apical membrane → hydrolyzed intracellularly
• Neonates: endocytosis of intact proteins (immunoglobulins from breast milk)

Fat Digestion

• Short-chain fatty acids (< C12): water-soluble → portal blood directly (NOT lymphatics)
• Long-chain fatty acids (> C12): require micelles + chylomicron route via lymphatics
• Bile salt reabsorption: 94-96% reabsorbed in terminal ileum (active transport) → portal vein → liver → enterohepatic circulation; pool recycles 2-3x per meal
• Fat-soluble vitamins (A, D, E, K): absorbed with fat via micelle/chylomicron route

Clinical relevance: Steatorrhea (fat malabsorption) occurs in: pancreatic exocrine insufficiency (no lipase), bile salt deficiency (cholestasis, terminal ileal disease/resection), celiac disease (villous atrophy), short bowel syndrome. Characterized by bulky, greasy, foul-smelling, floating stools.

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Absorption - Site Specificity (High-Yield Table)

Clinical relevance: Terminal ileal disease or resection (Crohn's disease, right hemicolectomy) → B12 deficiency (megaloblastic anemia, subacute combined degeneration of spinal cord) + bile salt malabsorption (cholerheic diarrhea or steatorrhea if extensive).

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PART 5: BILE AND THE LIVER

• Water (97%), bile salts (conjugated = glyco- or tauro-cholic acid), cholesterol, phospholipids (lecithin), bilirubin, electrolytes
• Volume: 600-1000 mL/day
• Stored + concentrated (10-20x) in gallbladder

1. Fat digestion: bile salts emulsify fat + form mixed micelles
2. Excretion: bilirubin, cholesterol, drugs, metabolites

• Stores and concentrates bile between meals
• CCK (released by fat + protein in duodenum) → gallbladder contraction + Sphincter of Oddi relaxation → bile into duodenum
• Fasting → gallbladder fills; eating → gallbladder empties

Clinical relevance: Gallstones (cholelithiasis): cholesterol stones (80%, from supersaturated bile) or pigment stones (hemolysis, cirrhosis). Courvoisier's law: palpable non-tender gallbladder + jaundice = malignant obstruction (NOT gallstones, which cause contracted fibrotic gallbladder). Murphy's sign: tenderness at gallbladder point on inspiration = acute cholecystitis.

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PART 6: COLONIC PHYSIOLOGY

1. Water and electrolyte absorption: colon can absorb up to 4-5 L/day; normally ~1-1.5 L arrives, 100-200 mL excreted
2. Na+ absorption: aldosterone-regulated in distal colon (same as kidney)
3. Short-chain fatty acid absorption: bacterial fermentation of undigested carbohydrates → butyrate (colonocyte fuel) + propionate + acetate
4. Storage of feces

• Segmentation contractions: haustra mix contents (slow)
• Mass movements: 1-3 times/day; propel large amounts of feces toward rectum
  • Initiated by gastrocolic reflex (eating → distension of stomach/duodenum → mass movements in colon) - basis of post-meal urge to defecate
  • Also by duodenocolic reflex

• Rectum normally empty; feces held in sigmoid by anorectal angle + sphincters
• Defecation reflex:
  1. Mass movement fills rectum → rectal distension → urge to defecate
  2. Intrinsic reflex (myenteric): peristalsis in sigmoid + rectum, IAS relaxes (RAIR - Recto-Anal Inhibitory Reflex)
  3. Extrinsic reflex (spinal, via sacral parasympathetics S2-S4): amplifies intrinsic reflex
  4. Voluntary decision: if appropriate → EAS relaxes + Valsalva maneuver → defecation; if inappropriate → EAS contracts → rectum accommodates → urge subsides
• Valsalva maneuver: closes glottis, contracts abdominal muscles → ↑ intra-abdominal pressure

Clinical relevance: Hirschsprung's: no RAIR; Fecal incontinence: EAS weakness (obstetric injury, MS, cauda equina); Constipation: slow transit, pelvic floor dysfunction, or anorectal.

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PART 7: GI MOTILITY PATTERNS

Migrating Motor Complex (MMC)

• Active during fasting (not after meals)
• Cycle every 90 minutes; initiated by motilin from M cells
• 3 phases:
  • Phase I: quiescence (~45 min)
  • Phase II: irregular contractions (~30 min)
  • Phase III: intense "burst" of contractions sweeping from stomach to ileum (~5-10 min) - clears residual food, bacteria, secretions
• Disappears after eating (replaced by fed pattern)
• Absent MMC → Small Intestinal Bacterial Overgrowth (SIBO) → malabsorption, bloating, diarrhea

Peristalsis

• Ascending excitation (proximal contraction) + Descending inhibition (distal relaxation)
• Mediated by ENS: serotonin (5-HT) from enterochromaffin cells initiates peristaltic reflex
• "Law of the gut" (Bayliss-Starling): contraction above, relaxation below the bolus

Clinical relevance: Serotonin (5-HT3) antagonists (ondansetron) block the peristaltic reflex in the gut → used as antiemetics. Metoclopramide is a dopamine antagonist that enhances GI motility (prokinetic).

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PART 8: ENTERIC NERVOUS SYSTEM (ENS)

• Contains ~100 million neurons (more than spinal cord)
• Two ganglionated plexuses:
  • Submucosal (Meissner's plexus): controls secretion and blood flow
  • Myenteric (Auerbach's plexus): controls motility (between circular and longitudinal muscle)
• Can function independently of CNS
• Receives modulating input from:
  • Parasympathetic (vagus + S2-S4): generally stimulatory (via ACh, VIP)
  • Sympathetic (T5-L2): generally inhibitory (via NA) - decreases motility, constricts sphincters
• Neurotransmitters: ACh (excitatory), VIP (inhibitory/relaxant), NO (relaxant), substance P, 5-HT (serotonin), neuropeptide Y

Gut-Brain Axis: 90% of vagal fibers are AFFERENT (gut → brain), not the other way. The gut senses its environment constantly and reports to the brain. IBS is partly a gut-brain axis dysregulation disorder.

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PART 9: HIGH-YIELD CLINICAL CORRELATIONS

Key Clinical Points for Exams

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QUICK REVISION SUMMARY TABLE

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Sources

• Guyton and Hall Textbook of Medical Physiology, 14th Edition - Units XII-XIII (GI Physiology)
• Ganong's Review of Medical Physiology, 26th Edition - GI Section
• Sleisenger and Fordtran's GI and Liver Disease, 11th Edition - Stomach + Duodenum Anatomy chapters
• Yamada's Textbook of Gastroenterology, 7th Edition - GI wall layers and histopathology
• Harrison's Principles of Internal Medicine, 22nd Edition - GI clinical correlations