Key Clinical and Diagnostic Features of Psoriasis

Clinical Presentation

Psoriasis vulgaris typically manifests as well‑demarcated erythematous plaques covered by silvery scale, most frequently on the scalp, elbows, knees and sacral region, although any skin area may be involved. 1
The disease includes several morphological subtypes—plaque (most common), guttate, palmoplantar, inverse and pustular forms. 3
Characteristic lesions are well‑defined plaques with sharp borders. 5
A silvery‑white scale rests on an erythematous base and is a hallmark of the condition. 2
Lesions are symmetrically distributed on extensor surfaces (e.g., elbows, knees). 6

Approximately 50 % of individuals with psoriasis develop nail changes, including pitting, onycholysis, subungual hyperkeratosis and the “oil‑drop” (salmon‑colored) sign. 8

Inverse psoriasis presents as erythematous plaques with minimal scaling in intertriginous zones (axillae, genital, perineal, intergluteal, inframammary areas). 6
Erythrodermic psoriasis is characterized by generalized erythema and scaling; a personal or family history of psoriasis and the presence of indurated, silvery‑scaled plaques on the trunk support the diagnosis. 10
Guttate psoriasis appears as numerous 1–10 mm salmon‑pink papules with fine scale, most common in patients younger than 30 years and typically located on the trunk and proximal limbs. 8

Psoriasis preferentially involves extensor surfaces, scalp and sacral area, whereas atopic dermatitis favors flexural sites. 1
Psoriatic plaques are well‑defined with thick silvery scale, contrasting with the poorly defined, eczematous, often lichenified lesions of atopic dermatitis. 3

Psoriatic lesions are symmetrical, well‑demarcated, and lack an elevated active border, while tinea corporis shows annular lesions with a raised border, central clearing and an asymmetrical pattern. 2
Potassium hydroxide (KOH) examination is negative in psoriasis but positive for hyphae in tinea corporis. 7
Mycological culture confirms fungal infection in tinea corporis, distinguishing it from psoriasis. 9

All statements are based on recommendations and observations from the American Academy of Dermatology.