A 38-year-old man from Delhi presents with a 6-month history of well-demarcated, erythematous plaques with silvery-white scale on his elbows, knees, and scalp. He reports that the lesions worsen in winter and improve in summer. On examination, positive Auspitz sign and Koebner phenomenon are noted. He has no joint pain. What is the most likely diagnosis?

Explanation

## Diagnosis: Plaque Psoriasis **Key Point:** Plaque psoriasis is the most common form of psoriasis, accounting for ~90% of all cases. It presents with well-demarcated, erythematous plaques topped with silvery-white micaceous scale, classically on extensor surfaces (elbows, knees) and scalp. ### Clinical Features Supporting Diagnosis | Feature | Plaque Psoriasis | Clinical Significance | |---------|------------------|----------------------| | **Morphology** | Well-demarcated erythematous plaques | Sharply demarcated from normal skin | | **Scale** | Silvery-white, micaceous | Pathognomonic appearance | | **Sites** | Extensor surfaces, scalp, nails | Characteristic distribution | | **Auspitz sign** | Pinpoint bleeding after scale removal | Pathognomonic for psoriasis | | **Koebner phenomenon** | New lesions at trauma sites | Positive in ~25% of psoriasis patients | | **Seasonal variation** | Worse in winter, better in summer | Classic pattern in many patients | **High-Yield:** The combination of **well-demarcated plaques + silvery scale + extensor distribution + Auspitz sign + Koebner phenomenon** is virtually diagnostic of plaque psoriasis. ### Pathophysiology 1. Hyperproliferation of keratinocytes (turnover time reduced from 28 to 3–4 days) 2. Infiltration of T lymphocytes and neutrophils into dermis and epidermis 3. Increased expression of TNF-α, IL-17, and IL-23 4. Elongation of rete ridges and thinning of suprapapillary epidermis **Clinical Pearl:** The Auspitz sign (pinpoint bleeding after gentle scraping of scale) is highly specific for psoriasis because the thinned suprapapillary epidermis allows capillaries to bleed easily. ### Why Plaque Psoriasis is Correct This patient has the **classic presentation**: well-demarcated plaques with silvery scale on extensor surfaces and scalp, positive Auspitz and Koebner signs, and seasonal variation. These are the hallmark features of plaque psoriasis. [cite:Fitzpatrick's Dermatology 9e Ch 8]