Clinical pathology: upper and lower respiratory system made easy exam oriented
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Here is your comprehensive, exam-oriented clinical pathology guide for the upper and lower respiratory system — organized by high-yield topics with key facts, mechanisms, pathogens, and clinical pearls.
🫁 Clinical Pathology: Respiratory System
Exam-Oriented Master Guide
PART 1 — UPPER RESPIRATORY TRACT
Anatomical Scope: Nasal cavity → Pharynx → Larynx → Upper trachea
1. Common Cold (Acute Coryza)
| Feature | Detail |
|---|---|
| Pathogen | Rhinovirus (>100 types) — most common; also Coronavirus, RSV, Adenovirus |
| Mechanism | Virus binds ICAM-1 receptors on nasal epithelium → local inflammation |
| Histology | Mucosal edema, mild neutrophil infiltrate, goblet cell hyperplasia |
| Clinical | Rhinorrhea, nasal congestion, sneezing, sore throat, low/no fever |
| Exam Pearl | Rhinovirus causes 30–50% of all URIs; no fever helps distinguish from influenza |
2. Pharyngitis
| Pathogen | Key Feature |
|---|---|
| Streptococcus pyogenes (GAS) | Most important bacterial cause; tonsillar exudate, uvular deviation, no cough |
| Neisseria gonorrhoeae | Sexually transmitted; think in adolescents |
| Corynebacterium diphtheriae | Grayish pseudomembrane, bull neck, risk of airway obstruction |
| Arcanobacterium haemolyticum | Scarlatiniform rash in young adults |
| EBV (Mono) | Exudative tonsillitis + lymphadenopathy + splenomegaly + atypical lymphocytes |
⚠️ Centor Criteria (GAS Pharyngitis):
- Tonsillar exudate
- Tender anterior cervical lymphadenopathy
- Fever
- Absence of cough
Score 3–4 → treat empirically with penicillin V (first-line)
Complications of untreated GAS pharyngitis:
- Rheumatic fever (molecular mimicry → carditis, polyarthritis, chorea)
- Post-streptococcal glomerulonephritis (immune complex deposition)
- Peritonsillar abscess
3. Sinusitis
| Type | Pathogens |
|---|---|
| Acute bacterial | S. pneumoniae (most common), H. influenzae, M. catarrhalis |
| Chronic | S. aureus, Pseudomonas, anaerobes, fungi (Aspergillus, Mucor) |
| Viral | Rhinovirus, Influenza — precede bacterial superinfection |
Pathophysiology: Obstruction of sinus ostia → mucociliary stasis → bacterial overgrowth → mucosal edema → purulent exudate
Imaging Pearl: Air-fluid levels and mucosal thickening on CT/X-ray
Complications: Orbital cellulitis, meningitis, cavernous sinus thrombosis, brain abscess
4. Epiglottitis
| Feature | Detail |
|---|---|
| Classic Pathogen | Haemophilus influenzae type b (HiB) — pre-vaccine era; now S. pneumoniae, S. aureus |
| Age | Children 2–7 (classic); now more in adults post-vaccination |
| Presentation | Drooling, dysphagia, dysphonia, stridor, "tripod" position |
| X-ray Sign | "Thumbprint sign" on lateral neck X-ray (swollen epiglottis) |
| Emergency | Do NOT examine oropharynx → may precipitate total obstruction |
| Treatment | Secure airway first; IV cefotaxime/ceftriaxone |
5. Croup (Laryngotracheobronchitis)
| Feature | Detail |
|---|---|
| Pathogen | Parainfluenza virus type 1 (most common) |
| Age | 6 months – 3 years |
| Presentation | Inspiratory stridor, barking cough, hoarseness, low-grade fever |
| X-ray Sign | "Steeple sign" on AP neck X-ray (subglottic narrowing) |
| Treatment | Dexamethasone (single dose), nebulized epinephrine for moderate-severe |
💡 High-Yield: Steeple sign = Croup; Thumbprint sign = Epiglottitis
6. Otitis Media (Related Upper Tract Pathology)
- Pathogens: S. pneumoniae (most common), H. influenzae, M. catarrhalis, S. aureus
- Mechanism: Eustachian tube dysfunction → bacterial ascent from nasopharynx
- Treatment: Amoxicillin first-line; amoxicillin-clavulanate for treatment failure
7. Nasal Polyps
- Inflammatory; seen in chronic allergic rhinitis, aspirin-exacerbated respiratory disease (Samter's triad), and cystic fibrosis
- Samter's Triad: Asthma + Nasal polyps + Aspirin sensitivity
- Histology: Edematous stroma with eosinophilic infiltrate
PART 2 — LOWER RESPIRATORY TRACT
Anatomical Scope: Lower trachea → Bronchi → Bronchioles → Alveoli → Pleura
1. Bronchitis
Acute Bronchitis
- Pathogens: Viral (90%) — Influenza, RSV, Rhinovirus; Bacterial: M. catarrhalis, H. influenzae, B. pertussis
- Histology: Mucosal edema, neutrophil infiltrate, epithelial desquamation
- Clinical: Productive cough >5 days; normal CXR
- Key: Do NOT give antibiotics routinely (viral etiology in most)
Chronic Bronchitis (Component of COPD)
- Definition: Productive cough for ≥3 months/year for ≥2 consecutive years (not due to another cause)
- Mechanism: Cigarette smoke → mucus gland hypertrophy → Reid index >0.5 (mucous gland thickness/bronchial wall thickness)
- Histology: ↑Reid index, goblet cell metaplasia, squamous metaplasia, neutrophil/macrophage infiltrate
- "Blue Bloater": Hypercapnia + hypoxemia → cyanosis + cor pulmonale; minimal dyspnea until late
2. Emphysema
| Type | Affected Area | Cause |
|---|---|---|
| Centriacinar (Centrilobular) | Central acinus (respiratory bronchioles) | Cigarette smoking |
| Panacinar | Entire acinus | α1-antitrypsin deficiency |
| Paraseptal | Distal acinus near septae/pleura | Spontaneous pneumothorax in young males |
| Irregular | Irregular; around scars | Post-inflammatory scarring |
Mechanism: Protease-antiprotease imbalance → elastase destroys alveolar walls → ↓elastic recoil → air trapping
"Pink Puffer": Severe dyspnea, barrel chest, pursed-lip breathing, minimal cyanosis; ↑AP diameter
Histology: Enlarged airspaces without fibrosis; loss of alveolar walls
3. Asthma
| Feature | Detail |
|---|---|
| Type | Reversible obstructive airway disease |
| Mechanism | TH2-mediated inflammation → mast cell degranulation → bronchoconstriction + mucus hypersecretion |
| Early phase | IgE-mediated: histamine, LTC4/D4/E4 (leukotrienes) → bronchoconstriction (within minutes) |
| Late phase | Eosinophils, neutrophils, IL-4/5/13 → airway remodeling (hours later) |
| PFTs | ↓FEV1, ↓FEV1/FVC, FVC normal; reversibility >12% post-bronchodilator |
Histology (Classic Exam):
- Curschmann spirals (mucus plugs)
- Charcot-Leyden crystals (eosinophil breakdown products)
- Subepithelial fibrosis
- Goblet cell hyperplasia
- Smooth muscle hypertrophy/hyperplasia
- Thickened basement membrane
Status Asthmaticus: Prolonged severe attack unresponsive to bronchodilators → medical emergency
4. Bronchiectasis
| Feature | Detail |
|---|---|
| Definition | Permanent, abnormal dilatation of bronchi/bronchioles |
| Causes | Cystic fibrosis (#1 in young), post-infectious (TB, Pseudomonas, S. aureus), Kartagener syndrome, allergic bronchopulmonary aspergillosis (ABPA) |
| Mechanism | Chronic infection/inflammation → destruction of bronchial wall → dilation |
| Clinical | Copious foul-smelling sputum, hemoptysis, digital clubbing |
| Imaging | "Tram-track" sign on CXR; "Signet ring sign" on CT (bronchus wider than adjacent artery) |
Cystic Fibrosis Pearls:
- CFTR mutation (ΔF508 most common) → defective Cl⁻ channel → thick mucus → chronic Pseudomonas infection → bronchiectasis
- Pancreatic insufficiency + sinusitis + male infertility
- Sweat chloride test >60 mEq/L = diagnostic
5. Pneumonia
Classification by Anatomy
| Type | Feature |
|---|---|
| Lobar pneumonia | Entire lobe consolidated; classically S. pneumoniae |
| Bronchopneumonia | Patchy peribronchial consolidation; S. aureus, H. influenzae, Klebsiella |
| Interstitial pneumonia | Diffuse alveolar wall inflammation; atypical/viral pathogens |
Stages of Lobar Pneumonia (Robbins Classic)
| Stage | Timing | Gross | Microscopy |
|---|---|---|---|
| Congestion | 1–2 days | Red-purple, heavy | Vascular congestion, edema, few bacteria |
| Red Hepatization | 2–4 days | Liver-like, firm, red | Fibrin + RBCs + PMNs filling alveoli |
| Grey Hepatization | 4–8 days | Firm, grey | Fibrin + PMNs; RBCs lysed |
| Resolution | 8+ days | Returns to normal | Macrophages, enzymatic digestion of exudate |
Pathogens by Clinical Setting
| Setting | Key Pathogens |
|---|---|
| Community-acquired (typical) | S. pneumoniae, H. influenzae, Klebsiella pneumoniae |
| Atypical (walking pneumonia) | Mycoplasma pneumoniae, Chlamydia pneumoniae, Legionella pneumophila |
| Hospital-acquired (HAP) | P. aeruginosa, S. aureus (MRSA), Klebsiella, E. coli |
| Aspiration | Anaerobes (Bacteroides fragilis), Klebsiella |
| Immunocompromised | PCP (Pneumocystis jirovecii), Aspergillus, CMV, Nocardia |
| Neonates | Group B Streptococcus, E. coli |
Atypical Pneumonia — Special Cases
| Organism | Key Feature |
|---|---|
| Mycoplasma pneumoniae | Cold agglutinins; "walking pneumonia"; treat with macrolide |
| Legionella pneumophila | Water-cooling towers; Pontiac fever; hyponatremia; diarrhea; urine antigen test |
| Chlamydia pneumoniae | Hoarseness; gradual onset; treat with doxycycline/macrolide |
| Coxiella burnetii (Q fever) | Animal contact; atypical; endocarditis risk |
| Klebsiella | Alcoholics/diabetics; "currant jelly sputum"; upper lobe cavitation |
6. Tuberculosis (TB)
Primary TB
- First exposure → Ghon focus (lower lobe subpleural) + hilar lymph node → Ghon complex
- Usually heals with calcification → Ranke complex (calcified Ghon complex + calcified node)
- In immunocompromised: progressive primary TB
Secondary (Reactivation) TB
- Location: Apex of upper lobe (high O2 tension favors Mycobacterium tuberculosis)
- Histology: Caseating (cheese-like) granuloma with central necrosis
- Langhans giant cells + epithelioid macrophages + lymphocytic cuff
- Complications: Cavitation, hemoptysis, miliary TB (hematogenous spread), Pott's disease (vertebral TB), lupus vulgaris
Miliary TB
- Hematogenous spread → "millet seed" granulomas in all organs
- High-risk: Immunocompromised, children, elderly
7. Lung Abscess
| Feature | Detail |
|---|---|
| Definition | Localized suppurative necrosis of lung parenchyma |
| Key Cause | Aspiration (most common) — anaerobes, mixed organisms |
| Location | Right lower lobe > right upper lobe (aspiration in supine = superior segment RLL) |
| Clinical | Fever, cough, foul-smelling sputum, clubbing if chronic |
| CXR | Thick-walled cavity with air-fluid level |
| Treatment | Clindamycin + metronidazole; prolonged antibiotics; drainage if needed |
8. Pleural Effusion
| Type | Causes | Key Lab |
|---|---|---|
| Transudate (protein <3 g/dL, LDH <200) | CHF, cirrhosis, nephrotic syndrome | Light's criteria not met |
| Exudate (protein >3 g/dL, LDH >200) | Pneumonia, TB, malignancy, PE | Light's criteria met |
| Empyema | Pus in pleural space; pH <7.2 | S. aureus, S. pneumoniae, Klebsiella |
| Chylothorax | Lymphatic obstruction (lymphoma, surgery) | Milky fluid, high triglycerides |
| Hemothorax | Trauma, malignancy | Blood |
Light's Criteria (exudate if ANY one met):
- Pleural protein/serum protein > 0.5
- Pleural LDH/serum LDH > 0.6
- Pleural LDH > 2/3 upper limit of normal serum LDH
9. Pneumothorax
| Type | Cause |
|---|---|
| Spontaneous (primary) | Young, tall, thin males; rupture of subpleural bleb |
| Spontaneous (secondary) | COPD, CF, PCP pneumonia |
| Tension pneumothorax | Tracheal deviation away from lesion; absent breath sounds; hypotension |
| Treatment | Small: observation; Large: needle decompression → chest tube |
10. Interstitial Lung Diseases (ILD)
| Disease | Key Feature |
|---|---|
| Idiopathic Pulmonary Fibrosis (IPF) | "Honeycomb" lung on CT; UIP pattern; poor prognosis; anti-fibrotics (pirfenidone, nintedanib) |
| Sarcoidosis | Non-caseating granulomas; bilateral hilar lymphadenopathy; elevated ACE; treat with steroids |
| Hypersensitivity Pneumonitis | Farmer's lung, pigeon breeder's disease; Type III + IV hypersensitivity |
| Silicosis | Silica exposure; birefringent particles; upper lobe nodules; ↑TB risk |
| Asbestosis | Lower lobe fibrosis; ferruginous bodies; ↑risk mesothelioma + lung adenocarcinoma |
| Berylliosis | Non-caseating granulomas (mimics sarcoidosis); beryllium exposure |
11. Lung Tumors
Malignant Tumors — High-Yield Comparison
| Feature | Squamous Cell Ca | Adenocarcinoma | Small Cell Ca | Large Cell Ca |
|---|---|---|---|---|
| Location | Central | Peripheral | Central | Peripheral |
| Smoking | Strong (+) | Weak (+); nonsmokers | Strong (+) | (+) |
| Origin | Squamous epithelium | Glandular epithelium | Neuroendocrine cells | - |
| Markers | CK5/6, p63, p40 | TTF-1, CK7, napsin-A | Synaptophysin, chromogranin, CD56, TTF-1 | Keratin (+) |
| Paraneoplastic | PTHrP (↑Ca²⁺) | - | SIADH (ADH), ACTH (Cushing), Lambert-Eaton | - |
| Histology | Keratin pearls, intercellular bridges | Acinar/glandular, mucin; Clara cells | Oat cells, nuclear molding, crush artifact | Large pleomorphic |
| Special | Cavitation | Most common in women/nonsmokers | Never resect (chemotherapy only) | Rapid growth |
Mesothelioma
- Cause: Asbestos exposure (50-year latency)
- Histology: Biphasic (epithelioid + sarcomatoid)
- Markers: Calretinin, WT-1, mesothelin — negative for CEA/CD15
- Poor prognosis
Carcinoid Tumors
- Well-differentiated neuroendocrine; chromogranin/synaptophysin +
- Carcinoid syndrome: Flushing, diarrhea, wheezing (serotonin-mediated; only with liver mets)
- Low-grade malignant; surgical resection curative
12. ARDS (Acute Respiratory Distress Syndrome)
| Feature | Detail |
|---|---|
| Definition | Acute onset, bilateral infiltrates, PaO2/FiO2 <300 (mild), <200 (moderate), <100 (severe); not explained by cardiac failure |
| Causes | Sepsis (most common), trauma, aspiration, pancreatitis, massive transfusion |
| Mechanism | Alveolar-capillary injury → increased permeability → protein-rich edema → hyaline membrane formation |
| Histology | Diffuse alveolar damage (DAD): hyaline membranes, type II pneumocyte proliferation, fibrosis |
| Treatment | Lung-protective ventilation: low tidal volume (6 mL/kg), PEEP, prone positioning |
| Berlin Definition | Within 1 week; bilateral opacities on imaging; not fully explained by effusions, lobar collapse |
13. Pulmonary Embolism (PE)
| Feature | Detail |
|---|---|
| Source | DVT (iliac/femoral veins) → saddle embolus at pulmonary bifurcation |
| Virchow's Triad | Stasis + Endothelial injury + Hypercoagulability |
| Clinical | Pleuritic chest pain, dyspnea, hemoptysis; tachycardia; hypoxemia |
| ECG | S1Q3T3 pattern; right heart strain; sinus tachycardia |
| CXR | Often normal; Hampton's hump (wedge-shaped infarct); Westermark sign (oligemia) |
| Gold standard diagnosis | CT pulmonary angiography (CTPA) |
| Treatment | Anticoagulation; thrombolysis if massive PE with hemodynamic compromise |
🗝️ HIGH-YIELD EXAM MNEMONICS
Respiratory Tract Pathogens — "3 S's of Sinusitis"
Strep pneumo, Staph aureus, Sinusitis bugs (+ H. flu, M. catarrhalis)
"HAMMAN" for IPF features
Honeycomb, Apical sparing (actually basilar), Mechanical ventilation risks, Anti-fibrotics, No cure
Stages of Lobar Pneumonia: "Congestive Red Grey Resolution"
Congestion → Red hepatization → Grey hepatization → Resolution
Lung Cancer Associations
- Central tumors: Squamous + Small Cell
- Peripheral tumors: Adeno + Large Cell
- Paraneoplastic hypercalcemia: Squamous cell (PTHrP)
- SIADH: Small cell
- Lambert-Eaton: Small cell
📊 QUICK COMPARISON TABLES
Obstructive vs. Restrictive Lung Disease (PFTs)
| Parameter | Obstructive | Restrictive |
|---|---|---|
| FEV1 | ↓↓ | ↓ |
| FVC | Normal/↓ | ↓↓ |
| FEV1/FVC ratio | ↓ (<0.7) | Normal or ↑ (>0.7) |
| TLC | ↑ (air trapping) | ↓ |
| RV | ↑ | ↓ |
| DLCO | ↓ (emphysema) | ↓ (ILD) |
Transudate vs. Exudate (Light's Criteria)
| Feature | Transudate | Exudate |
|---|---|---|
| Protein | <3 g/dL | >3 g/dL |
| LDH | <200 IU/L | >200 IU/L |
| Specific gravity | <1.015 | >1.018 |
| Glucose | Normal | Often ↓ |
| pH | Normal | Often ↓ (empyema) |
⚡ RAPID-FIRE EXAM FACTS
- Most common cause of community-acquired pneumonia → Streptococcus pneumoniae
- Most common cause of atypical pneumonia → Mycoplasma pneumoniae
- "Currant jelly" sputum → Klebsiella pneumoniae (alcoholics/diabetics)
- Cold agglutinins → Mycoplasma pneumoniae
- Thumbprint sign on X-ray → Epiglottitis (H. influenzae)
- Steeple sign on X-ray → Croup (Parainfluenza virus)
- Reid index >0.5 → Chronic bronchitis
- Caseating granulomas → TB (vs. non-caseating in sarcoidosis)
- Ghon complex → Primary TB (lower lobe focus + hilar node)
- Ferruginous bodies → Asbestosis
- Hyaline membranes in alveoli → ARDS (diffuse alveolar damage)
- Hampton's hump → Pulmonary embolism with infarction
- α1-antitrypsin deficiency → Panacinar emphysema
- Sweat chloride >60 mEq/L → Cystic fibrosis
- Calretinin positive → Mesothelioma
- TTF-1 positive → Adenocarcinoma (and small cell) of lung
- Carcinoid syndrome → Only occurs with liver metastases (serotonin not cleared)
- Best treatment for tension pneumothorax → Immediate needle decompression (2nd ICS, midclavicular line)
- Lung-protective ventilation in ARDS → Tidal volume 6 mL/kg ideal body weight
- Sarcoidosis elevated lab → ACE (angiotensin-converting enzyme)
Sources: Medical Microbiology 9e; Murray & Nadel's Textbook of Respiratory Medicine; Tietz Textbook of Laboratory Medicine 7e; Harrison's Principles of Internal Medicine 22EYou've run out of free credits. Sign up for a free account to continue using Orris.
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