Regarding the clinical features and microbiology of impetigo, all of the following statements are correct EXCEPT:

Question

Accepted Answer

D. Impetigo is a superficial skin infection that typically affects the stratum basale and extends into the dermis. ## Impetigo: Depth of Infection

**Key Point:** Impetigo is a *superficial* pyoderma confined to the **epidermis** — specifically the stratum corneum and superficial stratum granulosum. It does NOT extend into the dermis; this distinguishes it from deeper pyodermas like erysipelas and cellulitis.

### Correct Statements Explained

| Feature | Details |
|---------|----------|
| **Non-bullous form** | ~70% of cases; caused by *S. aureus* (increasingly MRSA) and *Streptococcus pyogenes* |
| **Bullous form** | Caused by *S. aureus* strains producing exfoliative toxins (exfoliatin A/B); toxins cleave desmoglein-1 in the granular layer |
| **Diagnosis** | Gram stain and culture of purulent exudate are gold standard; PCR can identify MRSA |

### Why Option 4 Is Wrong

**High-Yield:** Impetigo is **intraepidermal** — it remains within the epidermis and does NOT breach the dermal-epidermal junction. The bullae in bullous impetigo form at the level of the granular layer due to toxin-induced acantholysis. Involvement of the dermis would indicate a deeper infection (erysipelas, cellulitis, or abscess formation).

**Clinical Pearl:** This distinction is clinically important: impetigo typically lacks systemic symptoms (fever, lymphadenopathy are uncommon), whereas dermal involvement (erysipelas) presents with fever, chills, and regional lymphadenopathy.

[cite:Robbins 10e Ch 25]

Answer

A. The gold standard diagnostic test for impetigo is Gram stain and bacterial culture from the purulent exudate

Answer

B. Bullous impetigo is caused by exfoliative toxins (exfoliatin A and B) produced by Staphylococcus aureus

Answer

C. Non-bullous impetigo accounts for approximately 70% of cases and is caused predominantly by Staphylococcus aureus

Feature	Details
Non-bullous form	~70% of cases; caused by S. aureus (increasingly MRSA) and Streptococcus pyogenes
Bullous form	Caused by S. aureus strains producing exfoliative toxins (exfoliatin A/B); toxins cleave desmoglein-1 in the granular layer
Diagnosis	Gram stain and culture of purulent exudate are gold standard; PCR can identify MRSA

Regarding the clinical features and microbiology of impetigo, all of the following statements are correct EXCEPT:

Explanation

Impetigo: Depth of Infection

Correct Statements Explained

Why Option 4 Is Wrong

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