types of rashes in terms of polymorphic phase and monomorphic
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polymorphic monomorphic rash classification types skin lesions
Harrison's Principles of Internal Medicine, Twenty First Edition
Page 9935
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polymorphic monomorphic rash skin lesions types
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The retrieval returned content specifically about mastocytosis. Let me supplement this with broader knowledge on the full topic of polymorphic vs. monomorphic rashes across dermatology.
Rashes: Polymorphic vs. Monomorphic Phase
These terms describe whether the lesions in a rash present in multiple different morphological types simultaneously (polymorphic) or as a single, uniform lesion type (monomorphic).
Monomorphic Rashes
All lesions are of the same type, size, shape, and stage at any given time.
| Condition | Predominant Lesion Type | Notes |
|---|---|---|
| Acne vulgaris (comedonal) | Open/closed comedones | All at the same stage |
| Molluscum contagiosum | Pearly papules with central umbilication | Uniform throughout |
| Flat warts (verruca plana) | Flat-topped papules | Consistent morphology |
| Lichen planus | Violaceous, flat-topped papules | "5 Ps" — classic uniform look |
| Pityriasis rosea (herald patch phase) | Oval scaly plaques | Herald patch is typically solitary and uniform |
| Drug eruptions (morbilliform) | Maculopapular erythema | Widespread but uniform |
| Monomorphic MPCM | Small, uniform reddish-brown macules | Seen in adults; regular size/shape (see below) |
| Secondary syphilis | Copper-colored macules/papules | Relatively uniform per stage |
| Psoriasis (guttate) | Raindrop-shaped plaques | Uniform drop-like papules/plaques |
Polymorphic Rashes
Lesions appear in multiple morphological forms simultaneously — e.g., macules + papules + vesicles + crusts all present at once.
| Condition | Lesion Types Present Simultaneously | Notes |
|---|---|---|
| Varicella (chickenpox) | Macules → papules → vesicles → pustules → crusts | Classic "dew drop on rose petal"; all stages at once |
| Herpes zoster | Erythematous macules, papules, vesicles, crusts | Dermatomal; polymorphic over time |
| Eczema / Atopic dermatitis | Erythema, papules, vesicles, lichenification, excoriations | Acute and chronic lesions co-exist |
| Erythema multiforme (EM) | Macules, papules, targetoid lesions, bullae | Target/iris lesions are pathognomonic |
| Stevens-Johnson Syndrome / TEN | Macules, bullae, erosions, Nikolsky sign | Severe; highly polymorphic |
| Pemphigus vulgaris | Flaccid bullae, erosions, crusts | Bullae rupture → erosions coexist |
| Bullous pemphigoid | Urticarial plaques, tense bullae, erosions | Multiple morphologies simultaneously |
| Dermatomyositis | Gottron's papules, heliotrope rash, poikiloderma | Diverse morphology across body |
| Polymorphic MPCM | Macules, papules, plaques, nodules of varying sizes | Predominantly pediatric (see image below) |
| Polymorphic light eruption (PMLE) | Papules, papulovesicles, plaques | Variable between patients but consistent per patient |
Key Example: Polymorphic vs. Monomorphic MPCM
In cutaneous mastocytosis (MPCM), Harrison's Principles (p. 9935) distinguishes two variants:
- Polymorphic MPCM — reddish-brown lesions of varying size, shape, and morphology (macules, papules, large plaques/nodules); predominant in children
- Monomorphic MPCM — small, uniform, regular macules/papules; predominant in adults
All MPCM lesions share Darier's sign (urtication on stroking).
Conceptual Summary
| Feature | Monomorphic | Polymorphic |
|---|---|---|
| Lesion types | Single type | Multiple types simultaneously |
| Developmental stage | Same stage throughout | Different stages co-present |
| Classic example | Molluscum, flat warts, lichen planus | Varicella, EM, eczema |
| Diagnostic implication | Suggests a single, uniform pathologic process | Suggests an evolving or multi-mechanism process |
Clinical pearl: Varicella is the textbook polymorphic rash — at any one time you see macules, papules, vesicles, and crusts together. Lichen planus is the textbook monomorphic rash — flat-topped violaceous papules, all looking alike.
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