Infectious Diseases

Infectious Diseases

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Infectious disease pathology is the study of structural and functional alterations produced by bacteria, viruses, fungi, parasites, and prions, mediated through virulence factors, host inflammation, and characteristic patterns of tissue injury. Must-know concepts:

• Exotoxins are secreted proteins (often A–B subunit, heat-labile) — examples: diphtheria (inactivates EF-2), tetanus and botulinum (block neurotransmitter release), cholera (permanently activates Gs → ↑cAMP).
• Endotoxin (LPS / Lipid A) of Gram-negative bacteria triggers TLR4 → TNF-α, IL-1, IL-6 → septic shock, DIC, and acute respiratory distress syndrome.
• Patterns of inflammation: suppurative (pyogenic cocci, neutrophils), granulomatous (TB, leprosy, fungi — caseous necrosis + Langhans giant cells), lymphocytic/mononuclear (viral), and necrotizing (Clostridia, gas gangrene).
• TB morphology: Ghon complex (primary), apical cavitation (secondary), caseous necrosis, acid-fast bacilli on Ziehl–Neelsen stain.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Bacterial Pathogenesis Mechanisms

Adhesion (pili, MS fimbriae of E. coli), invasion (capsule anti-phagocytosis — Streptococcus pneumoniae, Klebsiella), and toxin release drive disease. Exotoxins are highly potent secreted proteins categorized as: (1) A–B subunit toxins (diphtheria, cholera, pertussis, E. coli LT), (2) membrane-damaging (α-toxin of Clostridium perfringens), and (3) superantigens (TSST-1 — toxic shock). Endotoxin (LPS) acts through CD14/TLR4 → MyD88 → NF-κB → cytokine storm, clinically presenting as gram-negative sepsis with hypotension and DIC.

Patterns of Inflammation

Pattern	Cells	Typical Organisms
Suppurative	Neutrophils + liquefactive necrosis	Staphylococci, Streptococci
Granulomatous	Epithelioid histiocytes + Langhans giant cells	M. tuberculosis, leprosy, fungi
Mononuclear/lymphocytic	Lymphocytes, plasma cells	Viruses, Treponema
Necrotizing	Coagulative/liquefactive with gas	Clostridium spp.
Pseudomembranous	Fibrin + necrotic debris	C. difficile

Mycobacteria

M. tuberculosis produces caseous necrosis with Langhans giant cells (peripheral horseshoe nuclei) and epithelioid granulomas, diagnosed by Ziehl–Neelsen or Fite stain for acid-fast bacilli. Leprosy spectrum: Tuberculoid (Th1-dominant, paucibacillary, lepromin-positive) → Borderline → Lepromatous (Th2-dominant, multibacillary, Virchow/foamy cells, lepromin-negative, Lucio phenomenon).

Syphilis Staging

Primary: painless chancre with treponemes on dark-field. Secondary: condylomata lata, copper maculopapular rash, plasma-cell infiltrates. Tertiary: gummas (granulomatous necrosis), obliterative endarteritis (endarteritis obliterans), neurosyphilis (tabes dorsalis, general paresis). Congenital: Hutchinson triad (notched incisors, interstitial keratitis, eighth-nerve deafness), saddle nose, saber shins, mulberry molars.

Viral Cytopathology

Inclusion bodies are diagnostic anchors: Negri bodies (rabies, hippocampal neurons, intracytoplasmic), Cowdry type A (HSV, intranuclear), Guarnieri bodies (smallpox), Warthin–Finkeldey (measles, Warthin giant cells in lymph node), Bollinger bodies (fowlpox), Molluscum bodies (molluscum contagiosum). Dane particle = complete infectious HBV virion with HBsAg envelope and HBcAg core.

Exam Pattern Focus

INI CET tests image-based identification of granulomas, inclusion bodies, and staining patterns in clinical scenarios. A single clinical stem (e.g., “40-year-old man with chronic cough, apical cavitation”) is paired with a histopathology image and asks for the staining method or morphology — practice 30+ such combined vignettes.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Edge Cases and Diagnostic Pitfalls

Granuloma vs. pseudogranuloma: foreign-body reactions lack caseous necrosis and giant-cell organization but can mimic TB on H&E — always correlate with Ziehl–Neelsen and culture. Lepromatous leprosy may show a negative lepromin test even after years of disease because of anergy; this is the opposite of tuberculoid leprosy and is a frequently tested distinction. Ghon focus vs. Ghon complex: the Ghon focus is the subpleural caseous lesion itself; the Ghon complex = focus + ipsilateral hilar lymphadenopathy. A Simon focus is a secondary TB implant in the apex during bacteremia of primary infection — confusing these on an image-based question costs marks.

Viral Hepatitis Patterns

HAV and HEV are directly cytopathic; HBV and HCV cause predominantly immune-mediated hepatocyte injury through CD8+ cytotoxic T-cell recognition of viral peptides on HLA class I. Patterns of necrosis include: focal (spotty) necroinflammation, piecemeal/interface necrosis (chronic active hepatitis, especially autoimmune and HCV), bridging necrosis (severe chronic hepatitis), panlobular (fulminant), and confluent. Dutcher bodies (intracytoplasmic Russell-body–like Ig inclusions in plasma cells) suggest lymphoplasmacytic neoplasm but may be confused with viral inclusions — always confirm with immunoglobulin immunostaining.

Fungal and Parasitic Pathology

Fungi stain with PAS (mucopolysaccharides) and Gomori methenamine silver (GMS) — both highlight Candida, Aspergillus, Cryptococcus, and Pneumocystis. Mucicarmine specifically stains the Cryptococcus capsule (red). Pneumocystis jirovecii shows cup-shaped cysts with central dot on GMS; serum LDH is the bedside clue. Plasmodium falciparum parasitized RBCs lack enlargement and show Maurer clefts (vs. Schüffner dots in P. vivax).

Bacterial Identification Methods

Quellung reaction (capsule swelling with type-specific antiserum) — S. pneumoniae, H. influenzae, Klebsiella. Lancefield grouping (cell-wall carbohydrate) classifies β-hemolytic streptococci; group A = S. pyogenes (bacitracin-sensitive, PYR-positive), group B = S. agalactiae (CAMP-positive). Cold agglutinins indicate Mycoplasma pneumoniae or infectious mononucleosis. Wayson stain highlights bipolar staining of Yersinia pestis.

Common Traps

1. Confusing exotoxin (secreted, immunogenic, can be toxoided — e.g., tetanus toxoid) with endotoxin (LPS, NOT toxoidable).
2. Calling Langhans giant cells “Touton giant cells” — Touton cells (foamy cytoplasm with peripheral nuclei) are seen in xanthomas and juvenile xanthogranuloma, NOT TB.
3. Misreading Reed–Sternberg cells (Hodgkin lymphoma) as infectious — CD15+, CD30+, EBV-associated in mixed cellularity subtype, but neoplastic, not infectious.
4. Stating “Langhans cells” of skin (dendritic APCs) when meaning Langhans giant cells of granulomas — these are unrelated entities.

Practice Prompts

Prompt 1: A 25-year-old HIV-positive man presents with a skin biopsy showing foamy macrophages packed with acid-fast bacilli on Fite stain, and a negative lepromin test. Identify the disease pole, expected CD4 count trend, and the histological hallmark cell name. Prompt 2: A 50-year-old alcoholic with cavitating apical lung lesion — outline the four histological layers from caseous center outward (caseous necrosis → epithelioid granuloma → Langhans giant cells → fibroblasts/lymphocytes) and explain why CD4+ Th1 cells and IFN-γ are central to containment.

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