## Distinguishing Plaque from Guttate Psoriasis **Key Point:** The cardinal morphological difference between plaque and guttate psoriasis is lesion size. Plaque psoriasis presents with large, well-demarcated plaques (typically >1 cm), while guttate psoriasis is characterized by small, drop-like papules (<1 cm). ### Comparative Table | Feature | Plaque Psoriasis | Guttate Psoriasis | | --- | --- | --- | | **Lesion size** | >1 cm (plaques) | <1 cm (papules) | | **Distribution** | Extensor surfaces, scalp, nails | Trunk, proximal limbs | | **Onset** | Insidious, chronic | Acute (weeks after streptococcal infection) | | **Trigger** | Koebner phenomenon common | Often follows URTI/strep throat | | **Course** | Persistent | May resolve or evolve to plaque type | | **Koebner phenomenon** | Present in ~25% | Present in ~25% | | **Family history** | Positive in 30–40% | Positive in 30–40% | **High-Yield:** Both types share identical histopathology (parakeratosis, regular acanthosis, dilated capillaries), identical genetic predisposition, and identical potential for Koebner phenomenon. **Size is the sole reliable clinical discriminator.** **Clinical Pearl:** Guttate psoriasis often follows Group A Streptococcal pharyngitis by 2–3 weeks. A history of acute onset with preceding sore throat strongly suggests guttate rather than plaque type. ### Why Other Features Do NOT Distinguish - **Koebner phenomenon:** Present in both types (~25% of patients each); not discriminatory. - **Family history:** Equally common in both; genetic predisposition is identical. - **Flexural involvement:** Plaque psoriasis typically spares flexures (inverse psoriasis is a separate variant); guttate favors trunk and proximal limbs—but this is distribution, not a sharp dividing line. 
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