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DR3.1 | Psoriasis Recognition and Differential Diagnosis — SDL Guide (Part 2)

Clinical Features and Diagnosis of Psoriasis

A four-panel medical diagram shows typical psoriasis distribution, plaque morphology with Koebner phenomenon, nail changes, psoriatic arthritis, and biopsy features. — **Panel A:** Typical psoriasis distribution: scalp hairline, extensor elbows, extensor knees, lumbosacral area, umbilicus, natal cleft, genital region. **Panel B:** Classic plaque morphology and Koebner phenomenon: well-demarcated erythematous plaque, silvery-white scale, linear lesion along scratch. **Panel C:** Psoriatic nail changes: nail pitting, onycholysis, subungual hyperkeratosis, orange-brown oil-drop sign. **Panel D:** Clinical and biopsy clues: distal interphalangeal joint swelling in psoriatic arthritis, parakeratosis, Munro microabscesses, thinning of suprapapillary epidermis, dilated tortuous capillaries.

Moving from a single plaque to the whole patient, a systematic examination both confirms psoriasis and gauges its extent. Begin with the distribution: psoriasis has a strong predilection for the extensor surfaces (elbows, knees), the scalp (often with thick adherent scale at the hairline), the lumbosacral area, the umbilicus, the natal cleft and the genitalia. Examine the nails specifically, because nail change is a powerful diagnostic clue and a marker of disease severity and arthritis risk: look for pitting (small thumbprint-like depressions), onycholysis (distal separation of the nail plate), subungual hyperkeratosis, and the orange-brown "oil-drop" sign. Ask about and examine for psoriatic arthritis — joint pain and swelling, classically of the distal interphalangeal joints — which co-exists in a significant minority. Demonstrate the Koebner phenomenon if there is a linear lesion along a scratch. The diagnosis remains overwhelmingly clinical; a skin biopsy is reserved for atypical cases and characteristically shows parakeratosis, Munro microabscesses (neutrophils in the horny layer), thinning of the suprapapillary epidermis, and dilated tortuous capillaries — the histological correlates of the bedside signs.

Labeled medical illustration of psoriatic nail changes showing nail pitting, onycholysis, subungual hyperkeratosis, and oil-drop sign with magnified detail panels. — **Panel A:** Overview of four psoriatic nails labeled with nail pitting, onycholysis, subungual hyperkeratosis, and oil-drop sign.. **Panel B:** Magnified nail plate showing punctate depressions labeled as nail pitting.. **Panel C:** Magnified distal nail showing white-yellow nail plate separation labeled as onycholysis.. **Panel D:** Side/cross-sectional distal nail detail showing thick subungual keratin debris labeled as subungual hyperkeratosis.. **Panel E:** Magnified nail showing salmon-yellow translucent discoloration labeled as oil-drop sign..

Checklist for the focused psoriasis examination:

Sites — extensors, scalp, umbilicus, natal cleft, genitalia.
Nails — pitting, onycholysis, oil-drop sign, subungual hyperkeratosis.
Joints — screen for psoriatic arthritis (especially DIP joints).
Signs — silvery scale, candle-grease and Auspitz on scraping, Koebner along trauma.

Differential Diagnosis of Psoriasis

A six-panel comparison diagram shows psoriasis and five major mimics with their key distinguishing clinical features. — **Panel A:** Psoriasis: well-demarcated erythematous plaque, silvery-white scale, extensor elbow distribution, Auspitz sign inset. **Panel B:** Seborrhoeic dermatitis: greasy yellowish scale, scalp margin, eyebrows, nasolabial folds, central chest, no silvery scale. **Panel C:** Pityriasis rosea: herald patch, oval secondary lesions, Christmas-tree trunk distribution, cleavage lines, self-limiting pattern. **Panel D:** Lichen planus: violaceous flat-topped polygonal pruritic papules, Wickham striae inset, no Auspitz sign. **Panel E:** Tinea corporis: annular lesion, central clearing, active scaly advancing edge, KOH mount showing fungal hyphae. **Panel F:** Secondary syphilis: coppery maculopapular rash, palm and sole involvement, positive serology.

Because several conditions produce red, scaly lesions, a disciplined differential is what separates a safe clinician from a careless one. The most important mimics each have a discriminating feature, and learning these pairs is the core skill of this module. Seborrhoeic dermatitis produces greasy, yellowish scale on seborrhoeic sites (scalp, eyebrows, nasolabial folds, central chest) and lacks the silvery scale and Auspitz sign. Pityriasis rosea begins with a single "herald patch" followed by smaller oval lesions in a "Christmas-tree" distribution along skin cleavage lines on the trunk, is self-limiting over weeks, and does not show Auspitz. Lichen planus — covered in detail in DR4.1 — presents with violaceous, flat-topped, polygonal pruritic papules bearing fine white Wickham striae, and crucially has no Auspitz sign. Tinea corporis (ringworm) shows an annular lesion with central clearing and an active scaly advancing edge, and is confirmed by a positive KOH mount showing fungal hyphae — the test that would have spared our hook patient months of mistreatment. Secondary syphilis must be remembered because, unlike most papulosquamous diseases, it characteristically involves the palms and soles and is confirmed serologically.

A labeled comparison chart contrasts psoriasis, seborrhoeic dermatitis, pityriasis rosea, and tinea corporis by scale appearance, distribution, special signs, and key investigations. — **Panel A:** Four-column comparison: Psoriasis with well-demarcated erythematous plaque, silvery-white candle-grease scale, extensor distribution, Auspitz sign, biopsy or ASO/throat swab for guttate trigger; Seborrhoeic dermatitis with greasy yellow scale, scalp/face/sternal distribution, dandruff and nasolabial involvement, clinical diagnosis; Pityriasis rosea with oval salmon patches and collarette scale, trunk Christmas-tree distribution, herald patch, clinical diagnosis; Tinea corporis with annular erythematous plaque, peripheral scaly advancing border and central clearing, exposed sites, positive KOH mount.. **Panel B:** Investigation callouts: KOH mount to exclude dermatophyte infection, skin biopsy for atypical morphology, ASO titre or throat swab when guttate psoriasis is suspected after sore throat..

For investigations in doubtful cases, reach for a KOH mount (to exclude tinea), a skin biopsy (for atypical morphology), and an ASO titre or throat swab when guttate psoriasis is suspected after a sore throat. Note that the formal graded grattage procedure is a separate bedside skill (DR3.2); for recognition, candle-grease scale plus Auspitz sign on careful scraping is sufficient.

SELF-CHECK

You see a patient with red scaly plaques and want to distinguish psoriasis from lichen planus at the bedside. Which single finding most reliably points AWAY from psoriasis and towards lichen planus?

A. Pinpoint bleeding (Auspitz sign) after scraping the scale

B. Silvery-white scale that flakes off like candle wax

C. Violaceous flat-topped papules with white lacy Wickham striae and no Auspitz sign

D. New lesions appearing along a scratch (Koebner phenomenon)

Reveal Answer

Answer: C. Violaceous flat-topped papules with white lacy Wickham striae and no Auspitz sign

Lichen planus is recognised by violaceous (purple), flat-topped, polygonal papules carrying fine white Wickham striae, and it does NOT produce the Auspitz sign. The Auspitz sign and silvery candle-grease scale are features of psoriasis, not lichen planus. The Koebner phenomenon is shared by both conditions (and by vitiligo), so it cannot be used to separate them — making the violaceous papule with Wickham striae and absent Auspitz the decisive discriminator.

Why Accurate Diagnosis Drives Management

Diagram showing how accurate recognition of psoriasis determines treatment tier and prevents unsafe systemic corticosteroid use that may trigger rebound pustular or erythrodermic flare. — **Panel A:** Clinical recognition of chronic plaque psoriasis with erythematous plaques, silvery scale, and assessment of extent of disease.. **Panel B:** Management ladder showing limited disease treated with topical therapy, moderate or topical-resistant disease with narrowband UVB phototherapy, and severe or disabling disease with systemic agents or biologics.. **Panel C:** Safety warning showing systemic corticosteroids contraindicated in psoriasis because withdrawal can precipitate rebound pustular or erythrodermic psoriasis..

It is worth closing the recognition loop by seeing why all this morphological care matters at the point of treatment, even though detailed treatment planning belongs to DR3.3. The variant and extent you diagnose select the management tier directly: limited chronic plaque disease is managed with topical agents, more extensive disease with phototherapy, and severe or disabling disease with systemic therapy. Most importantly, accurate recognition protects the patient from a specific and serious error. Systemic corticosteroids are contraindicated in psoriasis: although they may transiently improve the skin, their withdrawal can precipitate a rebound flare into generalised pustular or erythrodermic psoriasis, which is potentially life-threatening. The same caution applies to potent oral or injected steroids prescribed for unrelated reasons in a known psoriatic. Recognising psoriasis correctly is therefore not only a diagnostic exercise but a safety intervention — it tells you what to do and, just as vitally, what never to do.

The one-line management map (detailed in DR3.3):

Limited disease → topical therapy.
Moderate / topical-resistant → phototherapy (narrowband UVB).
Severe / disabling → systemic agents or biologics.
Never → systemic corticosteroids (rebound pustular/erythrodermic flare).