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AOBPa Pathology study materials
AOBPa Pathology study resources

AOBPa Pathology: Microbiology and Infectious Disease Pathology: Organism, Host, and Pattern

AOBPa Pathology · Work through your exam, one topic at a time.

All guide sections

Microbiology and Infectious Disease Pathology: Organism, Host, and Pattern

Infectious disease pathology questions connect an organism to a tissue pattern and then to a host context. The candidate should be able to move in both directions: from organism to lesion, and from lesion to likely organism.

Concepts to master

  • Bacterial patterns: suppurative (neutrophil-rich) inflammation for pyogenic bacteria; granulomatous for mycobacteria and fungi; pseudomembranous for C. difficile toxin.
  • Mycobacteria: caseating granulomas with Langhans giant cells; acid-fast staining (Ziehl-Neelsen) and the caveat that sensitivity is low, so culture or PCR may be needed.
  • Fungi: dimorphic (histoplasma, blastomyces, coccidioides), yeast (candida, cryptococcus - mucicarmine positive capsule), and mold (aspergillus - acute-angle branching septate hyphae; mucor - broad, nonseptate, right-angle).
  • Viruses: inclusion bodies (CMV owl's-eye, HSV Cowdry A, rabies Negri), koilocytes for HPV, and the immunocompromised host as a risk multiplier.
  • Parasites: malaria (ring forms, schizonts), toxoplasma (cysts in brain), and eosinophilia as a clue to helminths.

Traps

Stems often give an immunocompromised host and ask for the most likely organism - the answer shifts from community pathogens to opportunists. Do not confuse Aspergillus (septate, acute-angle) with Mucor (nonseptate, right-angle); this is a classic tested distinction. Also, a negative acid-fast stain does not exclude mycobacterial infection.

Rule of thumb

Use the host–pattern–organism triad: identify the immune status, name the histologic pattern, then narrow to organisms that produce that pattern in that host. The triad usually eliminates all but one option.

Worked example

A neutropenic patient with a pulmonary lesion shows broad, nonseptate hyphae branching at right angles with tissue invasion. The pattern is angioinvasive mold infection. The organism is Mucorales, not Aspergillus (which has septate, acute-angle hyphae). The trap answer is Aspergillus; the hyphal morphology and host settle it.