A 42-year-old woman with a 10-year history of psoriasis reports that her lesions most commonly appear on the extensor surfaces of her limbs and her scalp. Which is the most common site of involvement in psoriasis?

Explanation

## Most Common Sites of Psoriasis Involvement **Key Point:** The extensor surfaces of the elbows and knees are the most frequently affected sites in psoriasis, followed by the scalp, nails, and genitals. These areas account for the majority of initial and recurrent lesions. ### Typical Distribution Pattern | Site | Frequency | Characteristics | |------|-----------|------------------| | Extensor elbows/knees | Most common (>80%) | Well-demarcated plaques, easily visible | | Scalp | Very common | Silvery scale, may extend to hairline | | Nails | Common (10–50%) | Pitting, onycholysis, subungual hyperkeratosis | | Genitals | Common | Maceration, erosions, often missed | | Palms/soles | Less common | May be sole site (pustular variant) | | Flexural areas | Rare in plaque psoriasis | More typical of inverse psoriasis | ### Why Extensor Surfaces? 1. **Mechanical trauma:** Extensor surfaces experience more friction and pressure 2. **Koebner phenomenon:** Trauma triggers or worsens lesions 3. **Pressure sites:** Elbows and knees are high-pressure areas 4. **Evolutionary predisposition:** Genetic susceptibility manifests at these sites **High-Yield:** Remember the "**SENG**" mnemonic for common psoriasis sites: **S**calp, **E**xtensor surfaces, **N**ails, **G**enitals. **Clinical Pearl:** Inverse psoriasis (flexural involvement) is a distinct variant and is much rarer; it often presents with maceration and erosions rather than plaques, and is frequently misdiagnosed as fungal infection or intertrigo. **Warning:** Do not confuse plaque psoriasis (extensor) with inverse psoriasis (flexural); inverse psoriasis is uncommon and requires different management.