Core definition

Main morphologic patterns (bacterial)

1. Bronchopneumonia (lobular, patchy)
   • Patchy, often multilobar, commonly bilateral and basal consolidation
   • Centered on bronchi/bronchioles with spread to adjacent alveoli
   • Histology: neutrophil-rich suppurative exudate in bronchi, bronchioles, and alveolar spaces
2. Lobar pneumonia
   • Confluent consolidation of a large part of a lobe or an entire lobe
   • Classic sequence of 4 stages:
     • Congestion: heavy, boggy, red lung; vascular engorgement, edema, few neutrophils
     • Red hepatization: alveoli packed with neutrophils, RBCs, fibrin; firm “liver-like” lung
     • Gray hepatization: RBC breakdown with persistent fibrinopurulent exudate; gray-brown color
     • Resolution: enzymatic digestion and clearance of intraalveolar exudate (macrophage removal, expectoration, resorption; sometimes organization)

Viral pneumonia pathology

• Predominantly interstitial inflammation (alveolar septal widening/edema)
• Mononuclear infiltrate (lymphocytes, macrophages, occasional plasma cells)
• Alveoli may be relatively free early, then proteinaceous/cellular exudate may appear
• Severe injury can show diffuse alveolar damage with hyaline membranes (ARDS pattern)
• Superadded bacterial infection shifts to mixed or neutrophilic suppurative pattern

Common complications

• Lung abscess/necrosis
• Empyema (fibrinopurulent pleural involvement)
• Bacteremic spread causing metastatic infection (e.g., endocarditis, meningitis, brain/kidney/splenic abscesses)
• Pleural fibrosis/adhesions after organization

Etiology-pathology associations (high yield)

• S. pneumoniae: most common CAP; often lobar pattern
• Staph/Klebsiella/Pseudomonas: often severe necrotizing or suppurative disease in high-risk hosts
• Aspiration pneumonia: often polymicrobial, frequently necrotizing, abscess-prone