A 38-year-old man from Delhi presents with a 6-month history of well-demarcated, erythematous plaques with silvery-white scale over his elbows, knees, and scalp. He reports that the lesions worsen in winter and improve in summer. His nails show pitting and onycholysis. He denies systemic symptoms. On examination, positive Auspitz sign and Koebner phenomenon are noted. What is the most likely diagnosis?

## Diagnosis: Chronic Plaque Psoriasis ### Clinical Presentation **Key Point:** Chronic plaque psoriasis is the most common form of psoriasis, accounting for 80–90% of all cases. It presents with well-demarcated, erythematous plaques with characteristic silvery-white micaceous scale. ### Diagnostic Criteria Met | Feature | Finding in Case | Significance | |---------|-----------------|---------------| | Morphology | Well-demarcated erythematous plaques | Hallmark of plaque psoriasis | | Scale | Silvery-white, micaceous | Pathognomonic | | Distribution | Extensor surfaces (elbows, knees), scalp | Classic predilection sites | | Nail involvement | Pitting and onycholysis | Present in 10–50% of psoriasis patients | | Auspitz sign | Positive (bleeding when scale removed) | Indicates thinned suprapapillary epidermis | | Koebner phenomenon | Positive (lesions at trauma sites) | Seen in 25% of psoriasis patients | | Seasonal variation | Worse in winter, better in summer | Typical pattern | ### Pathophysiology 1. T-cell-mediated autoimmune response against keratinocytes 2. Increased epidermal proliferation (turnover in 3–4 days vs. normal 28 days) 3. Parakeratosis and thinning of suprapapillary epidermis 4. Dilated capillaries in dermal papillae → easy bleeding (Auspitz sign) **High-Yield:** The combination of **Auspitz sign + Koebner phenomenon + nail pitting + extensor distribution** is virtually diagnostic of psoriasis. ### Clinical Pearl **Clinical Pearl:** Nail changes in psoriasis include pitting (most common, 80% of cases), onycholysis, subungual hyperkeratosis, and nail dystrophy. These may be the only manifestation in some patients. ### Seasonal Pattern **Key Point:** Psoriasis typically worsens in winter (cold, dry air, reduced UV exposure) and improves in summer (increased UV-B exposure, which has immunosuppressive effects). [cite:Robbins and Cotran 10e Ch 25]