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Micro & Immunology: INBDE study notes and practice questions

This area connects the organisms of the mouth to disease and to infection control. It also covers the immune system: which cells and antibodies do what, and how hypersensitivity reactions are classified.

246 practice questions · 9 patient cases

Practice Micro & Immunology

Where it fits in the official INBDE blueprint

Diagnosis and Treatment Planning (infectious and immune disease). Nearest foundation knowledge areas: FK 5, FK 7. See the content outline for the official weights. This mapping is approximate: real items combine a clinical area with a foundation area.

What this area tests

High-yield checklist

Common traps

These notes are original study aids, not official exam content. Confirm clinical details against current guidelines.

Sample questions

1. Which fungal species is MOST common in the oral cavity?

  1. Aspergillus fumigatus
  2. Histoplasma capsulatum
  3. Cryptococcus neoformans
  4. Candida albicans
Show answer and explanation

Correct answer: D. Candida albicans

Candida albicans = by far most common oral fungus, responsible for oral candidiasis. Aspergillus = 2nd most common overall but rarely primary oral. Cryptococcus and Histoplasma = rare systemic fungi.

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2. What are the features of SCARLET FEVER?

  1. Caused by Staphylococcus aureus
  2. Strawberry tongue, white coating sloughing to red surface, Group A Streptococcus, contact/droplet transmission
  3. Characterized by vesicles on hands and feet only
  4. Caused by Herpes Simplex Virus
  5. Involves only the palate with white plaques
Show answer and explanation

Correct answer: B. Strawberry tongue, white coating sloughing to red surface, Group A Streptococcus, contact/droplet transmission

✓ B) Strawberry tongue + Group A Strep + contact/droplet transmission is correct.
Scarlet Fever: Causative agent = Streptococcus pyogenes (Group A beta-hemolytic Strep). Features: (1) Red rash (fine, sandpaper-like — 'scarlatiniform'); (2) STRAWBERRY TONGUE — white coating initially, then sloughs leaving hyperemic red fungiform papillae; (3) Sore throat, fever, swollen glands; (4) Transmission: contact and droplet; (5) Incubation: 1–3 days. Treatment: penicillin. Potential complications: rheumatic fever, post-streptococcal glomerulonephritis. Oral boards: know scarlet fever = Group A Strep + strawberry tongue.

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3. Which organism is intrinsically resistant to clindamycin, making it a poor empiric choice when this organism is suspected?

  1. Pseudomonas aeruginosa
  2. Streptococcus pyogenes
  3. Most strains of Staphylococcus aureus
  4. Peptostreptococcus
Show answer and explanation

Correct answer: A. Pseudomonas aeruginosa

Pseudomonas aeruginosa is intrinsically resistant to clindamycin — clindamycin's clinical spectrum (gram-positive cocci and most anaerobes) doesn't reliably cover gram-negative aerobic rods like Pseudomonas.

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4. What type of hypersensitivity reaction is Asthma (allergic/atopic asthma)?

  1. Type I (IgE-mediated)
  2. Type II (cytotoxic)
  3. Type III (immune complex)
  4. Type IV (delayed/cell-mediated)
Show answer and explanation

Correct answer: A. Type I (IgE-mediated)

A) Type I — CORRECT.
Allergic (extrinsic) asthma is a Type I (immediate) hypersensitivity reaction mediated by IgE antibodies on mast cells. Allergen exposure → IgE cross-linking → mast cell degranulation → histamine, leukotrienes, prostaglandins → bronchoconstriction, mucus production, airway inflammation.
B) Type II: Cytotoxic (e.g., hemolytic anemia, Goodpasture syndrome).
C) Type III: Immune complex deposition (e.g., lupus, serum sickness).
D) Type IV: Delayed cell-mediated (e.g., contact dermatitis, tuberculin test, Stevens-Johnson syndrome).

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5. What role does Fusobacterium nucleatum play in dental biofilm formation?

  1. It inhibits the growth of early colonizers
  2. It facilitates the attachment of late colonizers
  3. It has no significant role in biofilm formation
  4. It is primarily responsible for caries development
Show answer and explanation

Correct answer: B. It facilitates the attachment of late colonizers

Fusobacterium nucleatum is a KEY BRIDGING ORGANISM in dental biofilm. Early colonizers (Streptococcus species) bind to the pellicle. F. nucleatum serves as a BRIDGE — it co-aggregates with early colonizers AND facilitates the attachment of late colonizers (like Porphyromonas gingivalis, Treponema denticola) which are the major periodontal pathogens. It does NOT inhibit early colonizers (A wrong), it is very significant (C wrong), and it's not primarily cariogenic (D wrong). B is correct.

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6. Prolonged antibiotic therapy may predispose to which indigenous oral infection?

  1. Fusobacterium fusiforme
  2. Streptococcus mitis
  3. Treponema microdentium
  4. Actinomyces israelii
  5. Candida albicans
Show answer and explanation

Correct answer: E. Candida albicans

Prolonged antibiotics disrupt normal oral flora, allowing opportunistic overgrowth of Candida albicans u2014 causing oral candidiasis (thrush). Immunocompromised patients and those on broad-spectrum antibiotics are most at risk.

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7. What bacteria is primarily associated with ROOT SURFACE caries?

  1. Streptococcus mutans
  2. Porphyromonas gingivalis
  3. Actinomyces viscosus
  4. Lactobacillus casei
  5. Fusobacterium nucleatum
Show answer and explanation

Correct answer: C. Actinomyces viscosus

✓ C) Actinomyces viscosus is most commonly associated with root surface caries.
Root caries organisms: Actinomyces viscosus (and A. naeslundii) are gram-positive rods that preferentially colonize root surfaces (cementum/dentin). They produce acid at lower pH and can degrade the collagen matrix of cementum. S. mutans is also found in root caries but A. viscosus is THE most commonly tested organism specifically for ROOT surface caries. Lactobacillus is associated with lesion PROGRESSION (deeper dentinal caries). P. gingivalis = periodontal pathogen. Fusobacterium = bridging organism in biofilm.
  • A. ❌ P. gingivalis is a key periodontal pathogen (keystone pathogen) — not associated with caries.
  • B. ✅ CORRECT – Actinomyces viscosus is the primary pathogen in root surface caries, thriving in the exposed root environment.
  • C. ❌ S. mutans is the primary initiator of CORONAL (enamel) caries — not root surface caries.
  • D. ❌ Lactobacillus species are secondary colonizers that drive DENTINAL caries progression — not root surface caries initiation.

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8. In the plaque formation sequence, what forms first?

  1. Materia alba
  2. Pellicle
  3. Biofilm
  4. Calculus
Show answer and explanation

Correct answer: B. Pellicle

Dental plaque formation sequence: pellicle forms within seconds (glycoproteins, proline-rich proteins) u2192 bacteria attach within minutes forming biofilm u2192 calculus forms by mineralization of mature plaque over weeks. Materia alba is food/cell debris unrelated to plaque formation.

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Patient cases

All Micro & Immunology questions (246)