A 42-year-old man from Delhi presents with a 3-year history of well-demarcated, erythematous plaques with silvery-white scales on his elbows, knees, and scalp. He reports that lesions worsen in winter and improve in summer. On examination, Auspitz sign is positive. He has no joint pain or systemic symptoms. His nails appear normal. What is the most likely diagnosis?

## Clinical Diagnosis: Plaque Psoriasis **Key Point:** Plaque psoriasis is the most common form of psoriasis, accounting for 80–90% of all cases. It presents with well-demarcated, erythematous plaques topped with silvery-white scales, typically on extensor surfaces (elbows, knees) and scalp. ### Diagnostic Features in This Case | Feature | Finding | Significance | |---------|---------|---------------| | **Morphology** | Well-demarcated plaques | Classic for plaque psoriasis | | **Scale** | Silvery-white | Pathognomonic | | **Distribution** | Elbows, knees, scalp | Extensor predilection | | **Auspitz sign** | Positive (pinpoint bleeding on scale removal) | Indicates thinned suprapapillary epidermis | | **Seasonal pattern** | Worse in winter | Common trigger; UV light improves lesions | | **Systemic involvement** | None | No arthritis, no systemic features | | **Nail involvement** | Absent | Rules out severe or long-standing disease | ### Pathophysiology **High-Yield:** Psoriasis is a T-cell mediated autoimmune disorder with: - Th1 and Th17 cell infiltration into dermis - TNF-α, IL-17, IL-23 overproduction - Acanthosis (epidermal hyperplasia) - Parakeratosis (retention of nuclei in stratum corneum) - Elongation of rete ridges - Thinned suprapapillary epidermis (explains Auspitz sign) ### Why Plaque Psoriasis Fits Best 1. **Morphology & distribution** — Classic extensor plaques with silvery scale 2. **Auspitz sign positive** — Confirms thinned epidermis over dermal papillae 3. **No systemic features** — Excludes erythrodermic or pustular variants 4. **No guttate pattern** — Guttate psoriasis presents as small (< 1 cm), teardrop-shaped papules, often post-streptococcal 5. **Chronic course** — 3-year history consistent with chronic plaque type ### Clinical Pearl **Auspitz sign** (pinpoint bleeding on gentle scale removal) and **Koebner phenomenon** (lesions at sites of trauma) are hallmark signs of psoriasis. The Koebner phenomenon occurs in ~25% of psoriasis patients and is mediated by trauma-induced T-cell activation. ### Mnemonic: PLAQUES of Psoriasis - **P**laque (most common, 80–90%) - **L**ocalized to extensor surfaces - **A**cute onset or chronic course - **Q**uintessential silvery scale - **U**sually no systemic involvement (in plaque type) - **E**rythematous base - **S**cale removal → pinpoint bleeding (Auspitz) [cite:Fitzpatrick's Dermatology 9e Ch 8]