HPO Axis and Evolution of the Male & Female Reproductive Systems

Part 1: The HPO (Hypothalamic-Pituitary-Ovarian) Axis

The Three Levels

• FSH (Follicle-Stimulating Hormone) - stimulates follicle growth and estradiol production by granulosa cells
• LH (Luteinizing Hormone) - drives theca cells to produce androgens and triggers ovulation

The Menstrual Cycle: How the HPO Axis Runs It

Follicular / Proliferative Phase (Days 1-14)

• FSH and LH stimulate a cohort of ovarian follicles to grow
• All developing follicles produce estradiol, which gradually rises
• Rising estradiol stimulates rapid endometrial growth (proliferative phase)
• Early in this phase, estradiol exerts negative feedback - dampening GnRH/LH
• As estradiol rises further and stays elevated for >36-48 hours, it switches to positive feedback - a critical reversal that triggers the LH surge

The LH Surge and Ovulation

• The LH surge is an abrupt, dramatic rise around day 13-14, peaking ~12 hours after initiation and lasting ~48 hours
• Peak LH is roughly 3-fold above baseline
• The surge is triggered by persistently high estradiol, plus positive contributions from progesterone and activin
• The LH surge triggers ovulation: the dominant follicle ruptures and releases the oocyte

Luteal / Secretory Phase (Days 14-28)

• The ruptured follicle becomes the corpus luteum, which secretes both progesterone and estradiol
• Progesterone transforms the endometrium from proliferative to secretory (glandular phenotype) - preparing for implantation
• Progesterone + estradiol together exert strong negative feedback on GnRH/LH, suppressing new follicle development
• The corpus luteum also produces inhibin, which specifically suppresses FSH
• If fertilization does not occur, the corpus luteum degenerates (~day 26-28), progesterone/estradiol fall, negative feedback is removed, and menstruation begins - restarting the cycle

Key Feedback Summary

Medical Physiology (Boron & Boulpaep), p. 1629 - "This feedback loop is unusual because it elicits negative feedback on the hypothalamic-pituitary axis throughout most of the menstrual cycle but positive feedback immediately before ovulation."

Part 2: Evolution (Development) of the Male and Female Reproductive Systems

Step 1: Genetic Sex (Determined at Fertilization)

• XY = male; XX = female
• For the first ~5 weeks of gestation, gonads are indifferent (bipotential) - neither male nor female
• At ~week 7 in XY embryos, the SRY gene (sex-determining region of the Y chromosome) product activates, driving the indifferent gonad to develop into testes
• In XX embryos (no SRY), the gonads begin developing into ovaries slightly later (~week 9)

Step 2: Gonadal Sex

• Germ cells → spermatogonia
• Sertoli cells → produce Anti-Mullerian Hormone (AMH), also called Mullerian-Inhibiting Substance (MIS)
• Leydig cells → produce testosterone

• Germ cells → oogonia (arrested in meiosis I until ovulation)
• Granulosa cells → produce estradiol (with theca cells)
• Theca cells → produce progesterone and androgens (precursors for estradiol)

Step 3: Phenotypic Sex - The Duct Systems

• Wolffian (mesonephric) ducts - run from mesonephros to urogenital sinus
• Müllerian (paramesonephric) ducts - run parallel to wolffian ducts; fuse caudally to form the uterovaginal primordium

Male Development - Two Hormones Do the Work

• Epididymis (stores and matures sperm)
• Vas deferens (sperm transport)
• Seminal vesicles
• Ejaculatory duct

Female Development - Default Pathway

• No AMH → Müllerian ducts survive and differentiate into:
  • Fallopian tubes (oviducts)
  • Uterus
  • Cervix
  • Upper 1/3 of the vagina (lower 2/3 from urogenital sinus)
• No testosterone → Wolffian ducts degenerate (small remnants may persist as Gartner's duct cysts)

Step 4: External Genitalia - Common Origin, Different Fates

• In males, DHT (not testosterone itself) drives fusion of urogenital folds and enlargement of the phallus
• In females, the absence of androgens leaves the structures separate; the clitoris remains small
• External genitalia are anatomically indistinguishable until the end of the first trimester

Summary Table: Male vs. Female Differentiation

Puberty: HPO Axis "Wakes Up"

• Boys: GnRH → LH → Leydig cell proliferation → testosterone → spermarche, penile growth, voice change, facial hair, muscle mass
• Girls: GnRH → FSH/LH → estradiol from ovaries → breast budding (thelarche), then menarche ~2 years later; pubic/axillary hair (adrenarche) driven by adrenal androgens

Rheumatic Fever and Rheumatic Heart Disease - Comprehensive Pathology

1. Definition and Overview

2. Epidemiology

• Peak age of ARF: 5-15 years (peak years for streptococcal pharyngitis)
• ARF occurs equally in males and females, but RHD is more common in women
• Peak prevalence of RHD: 3rd and 4th decades (cumulative valve damage from recurrent attacks)
• Lifetime cumulative incidence after rheumatogenic GAS: 3-6% regardless of geography, implying a fixed proportion of genetically susceptible individuals
• Genetic heritability of RF estimated at ~60%; family history increases risk ~5x
• HLA class II associations, cytokine gene polymorphisms (TNF-α, TGF-β), and B-cell alloantigens have been implicated but findings are inconsistent

3. Pathogenesis - Molecular Mimicry

GAS Pharyngitis
      ↓ (2-3 week latent period)
Antibodies & CD4+ T cells against streptococcal M proteins
      ↓ (molecular mimicry)
Cross-react with cardiac antigens
(pericardial, myocardial, valvular proteins)
      ↓
Antibody-mediated + T cell-mediated cardiac injury

• N-acetyl-beta-d-glucosamine (GlcNAc) - group A carbohydrate epitope
• Laminin / laminar basement membrane in heart valve endothelium
• Cardiac myosin - cross-reactive with streptococcal M protein
• Collagen autoantibodies develop as collagen is released from damaged valves

Note: Streptococci are completely absent from the lesions - the damage is purely immunological.

4. Morphology (Pathologic Features)

4.1 Acute RF - The Aschoff Body (Pathognomonic)

• Focal granulomatous inflammatory lesions
• Composed of: T lymphocytes + occasional plasma cells + Anitschkow cells
• Anitschkow cells (caterpillar cells): plump activated macrophages with abundant cytoplasm and central ovoid nucleus in which chromatin condenses into a slender, wavy ribbon resembling a caterpillar - pathognomonic of RF
• Aschoff bodies can be found in any of the three cardiac layers (pancarditis)

4.2 Acute RF - Pancarditis

• Inflammation of the endocardium and left-sided valves
• Fibrinoid necrosis within valve cusps or tendinous cords
• Small (1-2 mm) verrucae (vegetations) form along the lines of closure of affected valves
• These are distinct from the large vegetations of infective endocarditis
• MacCallum plaques: irregular subendocardial thickenings in the left atrium, caused by regurgitant jets

4.3 Valve Involvement Pattern

4.4 Chronic RHD - The Healed Lesion

1. Leaflet thickening (fibrosis)
2. Commissural fusion - leads to stenosis; classically produces the "fish-mouth" or "buttonhole" appearance
3. Shortening of leaflets
4. Thickening and fusion of chordae tendineae

5. Clinical Features of Acute RF

Major Manifestations (Frequency)

6. Diagnosis - Revised Jones Criteria (2015, AHA)

Prerequisites (Evidence of Preceding GAS Infection)

• Positive throat culture or rapid strep antigen test
• Elevated or rising streptococcal antibody titer (ASO, anti-DNase B)

2015 Revised Jones Criteria

1. Subclinical valvulitis on echocardiography accepted as major criterion in all populations
2. Stratification into low-risk vs. moderate/high-risk populations
3. Monoarthritis and polyarthralgia added as major criteria in high-risk populations

Supporting Investigations

• ECG: Prolonged PR interval in only 35% of cases; higher-degree block is uncommon
• Echocardiogram: Recommended for ALL patients with confirmed/suspected ARF - detects subclinical carditis even without murmurs; monitor chronic RHD
• Labs: Elevated ESR/CRP; normochromic normocytic anemia; leukocytosis; elevated ASO titer

7. Natural History and Complications

• Patients with RF have ~50% risk of recurrence with subsequent untreated GAS pharyngitis
• Joint inflammation resolves fully (no residual damage), typically within 1 month without treatment
• Murmurs disappear in ~50% of patients with mild acute carditis without major cardiac enlargement
• Chronic valvular disease can develop even in patients who appeared to recover fully from the acute episode
• Sydenham chorea: Usually resolves in months; ~1/3 have recurrences
• Prognosis depends primarily on:
  • Severity of initial cardiac involvement
  • Whether episodes are recurrent (each attack causes cumulative valve damage)

8. Management

A. Acute Attack Treatment

• Single IM injection benzathine penicillin G (drug of choice) OR
• 10-day course of oral penicillin/amoxicillin OR
• Erythromycin (if penicillin-allergic)

• Aspirin (80-100 mg/kg/day) - first-line for arthritis and mild carditis; rapid symptomatic relief (NSAIDs also effective)
• Corticosteroids (prednisolone) - for severe carditis with cardiomegaly or heart failure; no proven benefit over aspirin for long-term valve outcomes
• Haloperidol / valproic acid - for Sydenham chorea

B. Secondary Prophylaxis (Most Important!)

• Benzathine penicillin G 1.2 million units IM every 4 weeks (every 3 weeks in high-risk patients) - most effective
• Alternative: Oral penicillin V 250 mg twice daily
• Penicillin-allergic: Oral macrolide (azithromycin 250 mg daily) or oral sulfadiazine

9. Chronic RHD - Valvular Consequences

Mitral Stenosis (Most Common)

• RHD is responsible for 99% of acquired mitral stenosis
• Commissural fusion → narrowed mitral orifice
• Left atrium enlarges → atrial fibrillation → left atrial thrombus → systemic embolism (especially stroke)
• Pulmonary hypertension develops over time
• Classic presentation: dyspnea, hemoptysis, mid-diastolic rumble at apex, opening snap

Mitral Regurgitation

• Can occur acutely (active carditis with valvulitis) or chronically (scarred, retracted leaflets)

Aortic Valve Disease

• Often co-exists with mitral disease
• Regurgitation more common than stenosis from RHD alone

Multi-Valve Disease

• RHD can involve multiple valves simultaneously, compounding hemodynamic compromise

Key Pathology Summary

Viva Questions: Rheumatic Fever & Rheumatic Heart Disease

SECTION 1: Basic Concepts

• Genetic susceptibility is required (heritability ~60%)
• HLA class II associations predispose certain individuals to cross-reactive immune responses
• The proportion of susceptible individuals is fixed at 3-6% globally regardless of geography
• Family history increases risk ~5x

SECTION 2: Pathogenesis

• Antibodies against streptococcal M protein cross-react with cardiac myosin
• Antibodies against GAS carbohydrate (GlcNAc) cross-react with laminin in heart valve endothelium
• T cells from peripheral blood and heart valves cross-react with both streptococcal M protein and cardiac myosin

1. First hit: Cross-reactive antibodies attack valve endothelium, causing inflammation and increased permeability
2. Second hit: This facilitates extravasation of cross-reactive T cells through the activated endothelium into valve tissue, leading to formation of Aschoff bodies (granulomatous nodules)

• GlcNAc, gangliosides, and dopamine receptors on neuronal cell surfaces in the brain
• These antibodies activate CaMKII (calcium/calmodulin-dependent protein kinase II) in neuronal cells
• They also recognize intracellular tubulin as a biomarker
• This targets the basal ganglia → hyperperfusion and increased metabolism → involuntary movements (chorea)

SECTION 3: Pathology / Morphology

• A central area of fibrinoid necrosis
• Surrounded by T lymphocytes and occasional plasma cells
• Anitschkow cells (caterpillar cells) - the pathognomonic cell type

• Abundant cytoplasm
• Central, round-to-ovoid nucleus (occasionally binucleate)
• Chromatin that condenses into a central, slender, wavy ribbon - resembling the body of a caterpillar when viewed in longitudinal section
• This ribbon-like chromatin pattern is the reason for the name "caterpillar cell"
• They are pathognomonic of RF and indicate active rheumatic myocarditis

• Pericarditis - fibrinous; resolves without permanent damage
• Myocarditis - Aschoff bodies in myocardium; rarely causes significant dysfunction
• Endocarditis - most clinically significant; causes valvulitis, verrucae formation, and ultimately leads to chronic RHD

1. Leaflet thickening (due to fibrosis)
2. Commissural fusion - fusion of adjacent leaflet margins → stenosis → "fish-mouth" or "buttonhole" appearance
3. Leaflet shortening - leaflets retract
4. Thickening and fusion of chordae tendineae - cords become thick, shortened, and fused together

1. Mitral alone - ~65-70% (most common)
2. Mitral + Aortic - ~25%
3. Tricuspid - infrequent (usually combined with mitral)
4. Pulmonary - rarely (right-sided valves protected by lower pressures)

SECTION 4: Clinical Features

• C - Carditis (clinical and/or subclinical echocardiographic valvulitis)
• A - Arthritis (migratory polyarthritis)
• S - Sydenham Chorea (St. Vitus dance)
• E - Erythema marginatum
• S - Subcutaneous nodules

• Fever ≥38.5°C
• Elevated ESR/CRP
• Prolonged PR interval on ECG
• Arthralgia (only if arthritis is NOT a major criterion)

• Migratory polyarthritis (moves from joint to joint, not additive)
• Affects large joints - knees, ankles, wrists, elbows (rarely hips and spine)
• Extremely painful, warm, swollen
• Synovial fluid is sterile with lymphocyte predominance
• Responds rapidly to aspirin/NSAIDs - failure to respond should prompt reconsideration of diagnosis
• Resolves completely in ~1 month without any residual joint damage

• Involuntary, non-rhythmic, purposeless movements of body, limbs, and face
• Usually more pronounced on one side (hemichorea)
• Disappears during sleep - distinguishes it from tics/tremors
• Milkmaid grip - inability to maintain sustained grip
• Pronator sign - when arms raised, hands pronate
• May occur in isolation weeks to months after the acute strep infection (latent period up to 6 months)
• Usually resolves in months; ~1/3 have recurrences
• Treated with haloperidol, valproic acid, or carbamazepine

• Flat or slightly raised, non-pruritic, non-scarring pink/red macules or papules
• Spreads in a serpiginous (snake-like) pattern, expanding outward with central clearing
• Located on the trunk and proximal limbs (NOT the face)
• Evanescent - can move and change hour to hour (the pen mark in clinical photos shows it moves ~60 min later)
• Does NOT scar
• Histology: perivascular neutrophilic and mononuclear infiltrates of the dermis

SECTION 5: Diagnosis

1. Subclinical valvulitis on echocardiography accepted as a major criterion (even without clinical murmur)
2. Risk stratification into low-risk vs. moderate/high-risk populations with different criteria thresholds
3. In moderate/high-risk populations: monoarthritis and polyarthralgia upgraded to major criteria; monoarthralgia added as minor criterion

• Positive throat swab culture for GAS
• Positive rapid streptococcal antigen test
• Elevated or rising ASO (Anti-Streptolysin O) titer - most commonly used
• Elevated anti-DNase B titer
• Note: in up to 1/3 of patients, no symptoms of preceding pharyngitis are reported - subclinical infection

1. Detects subclinical carditis in patients with no murmurs (now a major criterion)
2. Assesses severity of valvular involvement
3. Monitors patients with isolated Sydenham chorea (carditis may be silent)
4. Establishes baseline for monitoring RHD progression
5. Monitors recurrences

SECTION 6: Treatment and Prophylaxis

1. Suppress inflammation - minimize cardiac and joint damage
2. Eradicate GAS from the pharynx (even if culture negative)
3. Relieve symptoms - especially pain and fever
4. Initiate secondary prophylaxis - prevent future attacks and cumulative valve damage

• 10-day course of oral penicillin or amoxicillin
• Erythromycin if penicillin-allergic

• Each recurrence of ARF causes cumulative, additive valve damage
• ~50% of patients who have another untreated GAS infection get a new RF attack
• The chronic valvular disease (especially mitral stenosis) that causes death develops over years from repeated attacks
• Preventing recurrences is the single most effective intervention to reduce RHD mortality
• Even patients who recovered fully from acute RF with no valve disease can develop chronic RHD from subsequent attacks

SECTION 7: High-Yield / Tricky Questions

• Juvenile Idiopathic Arthritis (JIA) - especially systemic type (Still disease); distinguished by diurnal fever, evanescent rash, prolonged joint symptoms
• Lyme disease - Borrelia arthritis
• Reactive arthritis (post-infectious)
• Infective endocarditis - fever + valve lesions
• SLE - Libman-Sacks endocarditis
• Kawasaki disease (in children)
• Sickle cell arthropathy
• Leukemia/lymphoma - can present with joint pain and fever
• Serum sickness
• Gonococcal arthritis (in adolescents)

• Murmurs resolve in ~50% of patients with mild carditis and no major cardiac enlargement
• Without secondary prophylaxis, chronic RHD develops in a significant proportion
• Chronic RHD is the cause of 25-45% of all cardiovascular disease in developing countries and a major cause of heart failure
• Complications of established RHD include: atrial fibrillation, left atrial thrombus, systemic embolism (stroke), pulmonary hypertension, heart failure, and infective endocarditis
• Valve repair or replacement may eventually be required

High-Yield Examiner Favorites:

• Anitschkow cell = caterpillar cell = pathognomonic of RF
• Aschoff body = pathognomonic myocardial granuloma
• RHD = only cause of mitral stenosis
• Acute RF → mitral REGURGITATION; Chronic RHD → mitral STENOSIS
• Streptococci absent from heart lesions (immune-mediated, not infectious)
• Benzathine penicillin IM monthly = secondary prophylaxis cornerstone

Pharmacology of Hematinics

PART 1: IRON

Iron in the Body

• Oxygen transport (hemoglobin, myoglobin)
• Electron transport chain (cytochromes)
• DNA synthesis
• Enzyme function (catalase, peroxidase, xanthine oxidase)

Iron Metabolism and Absorption

• Adult male: ~1 mg/day
• Menstruating female: ~1.5-2 mg/day
• Pregnant female: ~3-4 mg/day
• Children/adolescents: +0.5 mg/day above adult

Mechanism of Iron Absorption (Step by Step)

Dietary iron (mainly Fe³⁺)
       ↓ Gastric acid + Duodenal cytochrome b (DcytB) — ferrireductase
Reduced to Fe²⁺
       ↓ DMT1 (Divalent Metal Transporter-1) on apical membrane
Enters enterocyte
       ↓ Stored as ferritin (if excess) OR exported via
Ferroportin-1 (FPN1) on basolateral membrane
       ↓ Hephaestin (ferroxidase) — re-oxidizes Fe²⁺ → Fe³⁺
Binds apotransferrin in plasma → Transferrin-bound iron
       ↓
Delivered to bone marrow, liver, other tissues

• Absorbed as intact heme moiety via Heme Carrier Protein-1
• More bioavailable than non-heme iron
• Once inside enterocyte, heme oxygenase releases Fe²⁺

• Ascorbic acid (Vitamin C) - reduces Fe³⁺ → Fe²⁺
• Acidic pH (gastric acid)
• Animal protein (heme iron)
• Iron deficiency state (upregulates DMT1, FPN1)
• Hypoxia

• Phytates (cereals, legumes)
• Tannins (tea, coffee)
• Oxalates, phosphates
• Antacids, PPIs (raise gastric pH)
• Tetracyclines, fluoroquinolones (chelate iron)
• Calcium (competes with DMT1)
• Excess iron (hepcidin mediated)

Hepcidin - Master Regulator of Iron

• Binds to Ferroportin-1 (FPN1) → causes its internalization and degradation
• This blocks iron export from enterocytes, macrophages, and hepatocytes
• Net effect: reduces circulating iron

Iron Deficiency Anemia (IDA)

• Low serum iron
• Low ferritin (first to fall - most sensitive early marker)
• High TIBC (Total Iron Binding Capacity)
• Low transferrin saturation (<16%)
• Microcytic, hypochromic RBCs on smear
• Low MCV (<80 fL)

• Fatigue, koilonychia (spoon nails), angular stomatitis, glossitis
• Pica (craving for ice, clay, starch)
• Restless legs syndrome
• Impaired cognitive/behavioral development in children

Oral Iron Preparations

• Fe²⁺ is more soluble at physiologic pH
• Directly transported by DMT1
• Fe³⁺ must first be reduced to Fe²⁺ before absorption

• Take on empty stomach (better absorption)
• Take with Vitamin C (enhances absorption)
• Avoid with milk, antacids, tea, coffee, tetracyclines
• If GI intolerance, take with food (reduces absorption ~40%)

• Reticulocytosis appears in 3-4 days (children) or >1 week (adults) - first sign of response
• Hemoglobin rises by 1-2 g/dL per month
• Continue 3-6 months AFTER hemoglobin normalizes to replenish stores

• Nausea, vomiting, epigastric discomfort
• Constipation (most common) or diarrhea
• Black/dark stools (warn patients - can mimic melena)
• Staining of teeth with liquid preparations (use straw)

Parenteral Iron Preparations

• Oral iron not tolerated
• Malabsorption (celiac, IBD, post-gastrectomy)
• Chronic kidney disease (CKD) - hepcidin blocks oral absorption
• Non-compliance
• Rapid iron repletion needed (e.g. pre-surgery)
• Chronic blood loss exceeding oral replacement capacity

• Anaphylaxis (serious - requires test dose + monitoring for 1 hour post-infusion)
• Delayed hypersensitivity: fever, malaise, arthralgia, lymphadenopathy (days-weeks later)
• IM injection risk: local pain, tissue necrosis, potential malignant change at injection site - IM route now largely abandoned
• Use extreme caution in rheumatoid arthritis (increased risk of delayed reactions)
• Withhold if plasma ferritin >800 μg/L (iron overload risk)

Iron Toxicity / Overdose

• Phases: GI symptoms → apparent recovery → shock and hepatic failure → recovery or death
• Mechanism: Fe²⁺ generates free radicals (Fenton reaction) → lipid peroxidation → cell death
• Treatment: Gastric lavage + Deferoxamine (chelation therapy) - binds free iron → excreted as ferrioxamine in urine (pink-orange discoloration)

PART 2: FOLIC ACID (Vitamin B9)

Biochemistry

Dietary Sources and Requirements

• Rich sources: fresh green vegetables (spinach, broccoli, asparagus), liver, yeast, fruits
• Cooking destroys up to 90% of folate
• Daily requirement: 400 μg/day (adults); 600 μg/day (pregnancy); 500 μg/day (lactation)
• Body stores: ~5-20 mg (liver) - depleted in 3-4 months of deficiency (compared to years for B12)

Causes of Folate Deficiency

1. Inadequate intake - poor diet, alcoholism (most common cause)
2. Increased demand - pregnancy, hemolytic anemia, rapid cell turnover
3. Malabsorption - celiac disease, tropical sprue
4. Drug-induced - methotrexate (DHFR inhibitor), trimethoprim, phenytoin, sulfasalazine
5. Alcohol - impairs absorption AND utilization

Pharmacological Uses of Folic Acid

• Dose: 5 mg/day orally for 4 months (or until dietary issues corrected)
• Parenteral if malabsorption

• Folic acid supplements reduce NTD risk by ~50-70%
• Periconceptional supplementation: 400 μg/day starting 1 month BEFORE conception and for first 12 weeks
• High-risk (previous NTD child): 4-5 mg/day

• 600 μg/day throughout pregnancy

• Use folinic acid (leucovorin/citrovorum factor) - bypasses DHFR inhibition
• This is the "leucovorin rescue" in cancer chemotherapy

Important Warning: Folate vs. B12 in Megaloblastic Anemia

NEVER give folic acid alone in suspected B12 deficiency without ruling out B12 deficiency first!

• Folic acid corrects the hematological picture (anemia) in B12 deficiency
• BUT it does NOT prevent or may worsen the neurological damage (subacute combined degeneration of the spinal cord)
• Always check both folate AND B12 levels before treating megaloblastic anemia

PART 3: VITAMIN B12 (Cobalamin)

Forms

• Cyanocobalamin - stable, synthetic, pharmaceutical form; converted to active forms in body
• Hydroxocobalamin - injectable; longer lasting (preferred for IM); better tissue retention
• Methylcobalamin - active form in cytoplasm
• Adenosylcobalamin - active form in mitochondria

Biochemical Roles

• Cofactor: Methylcobalamin
• Failure → "methyl-folate trap" → functional folate deficiency → megaloblastic anemia
• Elevated homocysteine → cardiovascular risk

• Cofactor: Adenosylcobalamin
• Failure → methylmalonic acid (MMA) accumulates → myelin damage → subacute combined degeneration of spinal cord
• MMA is a specific marker of B12 deficiency (elevated in B12 but NOT folate deficiency)

Absorption of B12 (Critical Pathway)

Dietary B12 (bound to food proteins)
      ↓ Pepsin + HCl (stomach)
Released from food
      ↓ Binds R-protein (haptocorrin, from saliva/gastric juice)
B12-R complex
      ↓ Pancreatic proteases in duodenum degrade R-protein
Free B12
      ↓ Binds Intrinsic Factor (IF) secreted by gastric parietal cells
B12-IF complex
      ↓ Absorbed at specific receptors in TERMINAL ILEUM (cubilin receptor)
Enters portal blood
      ↓ Binds transcobalamin II (TC II) for transport to tissues

Causes of B12 Deficiency

Clinical Features of B12 Deficiency

1. Megaloblastic anemia - hypersegmented neutrophils (>5 lobes), macro-ovalocytes, pancytopenia
2. Neurological: Subacute Combined Degeneration (SCD) of spinal cord:
   • Posterior columns (vibration, proprioception loss)
   • Lateral columns (UMN signs - spasticity, hyperreflexia)
   • Peripheral neuropathy
   • Dementia, psychiatric symptoms
3. Glossitis (beefy red, smooth tongue)
4. Elevated homocysteine AND methylmalonic acid (diagnostic)

Treatment of B12 Deficiency

• Hydroxocobalamin 1000 μg IM - loading: daily for 1 week, then weekly for 4 weeks, then monthly for life
• Cyanocobalamin 1000 μg IM as alternative

• Oral cyanocobalamin 1000-2000 μg/day - effective even without IF through passive diffusion (~1% absorbed passively)
• Intranasal cyanocobalamin available for maintenance

• Reticulocytosis peaks at 5-7 days
• Hemoglobin normalizes in 6-8 weeks
• Neurological improvement begins in weeks but may be incomplete if severe

PART 4: ERYTHROPOIESIS-STIMULATING AGENTS (ESAs)

• Hypertension (increased blood viscosity)
• Thrombosis/stroke (especially if over-correction)
• Pure red cell aplasia (anti-EPO antibodies - rare)
• Iron deficiency develops rapidly (increased demand) - must supplement iron concurrently

SUMMARY TABLE: Hematinics at a Glance

Sources: Goodman & Gilman's Pharmacological Basis of Therapeutics 13th Ed., Ch. 45 | Sleisenger & Fordtran's GI and Liver Disease | Rosen's Emergency Medicine | Comprehensive Clinical Nephrology 7th Ed.