A 38-year-old man presents with well-demarcated erythematous plaques on the extensor surfaces of elbows and knees, with silvery scales and pinpoint bleeding on scale removal. His nails show pitting. Which single feature best distinguishes chronic plaque psoriasis from psoriasiform dermatitis?

## Distinguishing Chronic Plaque Psoriasis from Psoriasiform Dermatitis ### The Auspitz Sign as the Best Discriminator **Key Point:** The Auspitz sign — pinpoint bleeding immediately upon removal of scale — is considered **pathognomonic** for psoriasis and does NOT occur in psoriasiform dermatitis or other papulosquamous disorders. Among all the listed features, it is the single most specific clinical sign. ### Pathophysiologic Basis The Auspitz sign reflects the characteristic histology of psoriasis (Fitzpatrick's Dermatology, 9e): - **Dilated, tortuous capillaries** in the elongated dermal papillae - **Thinned suprapapillary epidermis** (loss of the granular layer — hypogranulosis) - When scale is removed, the thin epidermis is breached, exposing these dilated vessels → pinpoint bleeding In psoriasiform dermatitis (e.g., seborrheic dermatitis, chronic eczema, pityriasis rosea), the dermal vasculature is not similarly dilated and the suprapapillary epidermis is not thinned, so the Auspitz sign is **absent**. ### Why Auspitz Sign Outperforms Nail Pitting as a Discriminator | Feature | Psoriasis | Psoriasiform Dermatitis | |---------|-----------|------------------------| | Silvery scales | Present | Often present (e.g., seborrheic dermatitis) | | Well-demarcated plaques | Yes | Can occur (e.g., chronic eczema) | | Nail pitting | Characteristic (~50% of cases) | Absent in most, but can occur in alopecia areata, eczema | | **Auspitz sign** | **Pathognomonic** | **Absent** | **Nail pitting**, while highly suggestive of psoriasis, has two important limitations as a discriminator: 1. It is present in only ~50% of psoriasis patients — it is **not universal**. 2. It can occasionally be seen in **alopecia areata** and **eczema**, reducing its specificity. The Auspitz sign, by contrast, is directly tied to the unique histopathology of psoriasis (suprapapillary thinning + dermal papillary vascular dilation) and is **not replicated** in psoriasiform dermatitis. **Clinical Pearl:** Silvery scales and well-demarcated plaques are shared features of psoriasis and many psoriasiform conditions, making them poor discriminators. The Auspitz sign is the single most specific *clinical* sign for psoriasis. **High-Yield:** In NEET PG / INI-CET, the Auspitz sign is repeatedly tested as the pathognomonic clinical sign for psoriasis. Nail pitting is a supportive feature but lacks the specificity of the Auspitz sign when distinguishing psoriasis from psoriasiform dermatitis. [cite: Fitzpatrick's Dermatology 9e Ch 8; Rook's Textbook of Dermatology 9e]