Step-by-step visual differentiation of psoriasis and eczema for NEET PG: Auspitz sign, Koebner phenomenon, silvery scales vs vesicles and lichenification, with diagnostic criteria, clinical images analysis, and practice MCQs.
Version 1.0 — Published April 2026
Psoriasis and eczema are the two most commonly confused dermatological conditions in NEET PG image-based questions. To differentiate them on sight:
A 35-year-old male bank employee presents to the dermatology OPD with multiple raised, reddish patches on both elbows and knees that have been present for 8 months. He reports that the patches have gradually increased in size. He describes moderate itching, worsening in winter months. He noticed that a scratch from his cat 3 weeks ago has developed a similar raised patch along the scratch line. He has no history of asthma, allergic rhinitis, or atopic dermatitis in childhood.
Examination findings:
On inspection of the elbows and knees, the following features are visible:
On gentle scraping of the scale with a glass slide:
A 35-year-old male presents with well-demarcated, erythematous plaques covered with silvery scales on both elbows and knees for 8 months. On removing the scales, pinpoint bleeding is observed. New similar lesions have appeared at the site of a recent scratch. The most likely diagnosis is:
Take a moment to work through this before reading the analysis below.
Systematic dermatological image reading prevents the common error of pattern-matching on color alone. Use this 5-step approach for every dermatology image MCQ:
This content is for educational purposes for NEET PG exam preparation. It is not a substitute for professional medical advice, diagnosis, or treatment. Clinical information has been reviewed by qualified medical professionals.
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Join on Telegram →The plaques have sharply defined, well-demarcated borders — you can clearly see where the lesion ends and normal skin begins. This is a hallmark of psoriasis. In contrast, eczema has ill-defined, gradually blending borders that merge with surrounding normal skin.
Why this matters: Border definition alone narrows the differential significantly. Well-demarcated plaques point to psoriasis, lichen planus, or fungal infection. Ill-defined patches point to eczema, seborrheic dermatitis, or early mycosis fungoides.
The scales are thick, silvery-white, and adherent. They form multiple layers (micaceous scale) that separate like candle wax (candle grease sign/Grattage test). This is characteristic of psoriasis.
Compare with:
The lesions are on extensor surfaces (elbows and knees) — the classic psoriasis distribution. Other psoriasis-favored sites: scalp (extending beyond the hairline), sacral area, umbilicus, nails.
| Distribution | Condition |
|---|---|
| Extensors (elbows, knees) | Psoriasis |
| Flexures (antecubital, popliteal fossae) | Atopic eczema |
| Wrists, ankles, genitalia, oral mucosa | Lichen planus |
| Nasolabial folds, scalp, eyebrows | Seborrheic dermatitis |
| Trunk (Christmas tree pattern) | Pityriasis rosea |
Three signs are virtually diagnostic of psoriasis:
Auspitz sign — pinpoint bleeding on scale removal. Caused by suprapapillary thinning of the epidermis and dilated capillary loops in elongated dermal papillae. This sign is positive in the vignette.
Koebner phenomenon (isomorphic response) — new psoriatic lesions at sites of trauma. The vignette describes a new plaque along a scratch mark. This occurs in active disease, typically 10-14 days after trauma. Also seen in lichen planus, vitiligo, and warts — but combined with silvery scales and extensor distribution, it confirms psoriasis.
Candle grease sign (Grattage test) — scales separate in layers like candle wax on scraping.
Answer: (b) Psoriasis vulgaris. The well-demarcated plaques with silvery scales on extensors, positive Auspitz sign, and Koebner phenomenon are pathognomonic.
The following table is the core differentiator that NEET PG image MCQs test:
| Feature | Psoriasis | Eczema (Atopic Dermatitis) |
|---|---|---|
| Borders | Well-demarcated, sharp | Ill-defined, blending |
| Scale | Thick, silvery-white, adherent | Fine, dry; or weeping/crusting (acute) |
| Surface | Dry, non-vesicular | Vesicles (acute), lichenification (chronic) |
| Color | Salmon-pink | Erythematous to brownish |
| Distribution | Extensors, scalp, nails, sacral | Flexures, face (infants), hands |
| Pruritus | Mild to moderate | Intense (hallmark symptom) |
| Auspitz sign | Positive | Negative |
| Koebner phenomenon | Present in active disease | Not typical |
| Age of onset | Bimodal: 20-30 and 50-60 years | Childhood onset (before age 5 in 85%) |
| Family history | Psoriasis, autoimmune diseases | Atopy (asthma, allergic rhinitis, eczema) |
| Nail changes | Pitting, oil-drop sign, onycholysis | Absent or non-specific |
| Histology | Munro microabscesses, regular acanthosis, suprapapillary thinning | Spongiosis, irregular acanthosis |
| Course | Chronic, relapsing-remitting | Chronic with acute flares |
| Condition | Key visual feature | Classic distribution | Differentiating sign |
|---|---|---|---|
| Psoriasis | Silvery scales, well-demarcated plaques | Extensors, scalp, nails | Auspitz sign, Koebner phenomenon |
| Eczema | Ill-defined, vesicular/lichenified | Flexures | Intense itch, atopic history |
| Seborrheic dermatitis | Greasy yellow scales | Scalp, nasolabial folds | Cradle cap in infants |
| Lichen planus | Violaceous flat-topped papules, Wickham striae | Wrists, ankles, oral mucosa | 6 Ps, oral erosions |
| Pityriasis rosea | Herald patch, then collarette scaling | Trunk (Christmas tree pattern) | Self-limiting (6-8 weeks) |
The Auspitz sign is the appearance of pinpoint bleeding when silvery scales are removed from a psoriatic plaque. It occurs because the psoriatic epidermis is thin above the dermal papillae (suprapapillary thinning), and removing the scales exposes dilated capillary loops. This sign is pathognomonic for psoriasis and does not occur in eczema.
The Koebner phenomenon (isomorphic response) is the development of new lesions at sites of skin trauma. In psoriasis, new plaques appear along scratch marks or scars, typically 10-14 days after trauma. Other conditions showing this include lichen planus, vitiligo, and warts.
Five visual keys: border definition (sharp in psoriasis, blurred in eczema), scale type (silvery in psoriasis, fine/absent in eczema), surface (dry in psoriasis, vesicular/lichenified in eczema), distribution (extensors vs flexures), and associated signs (Auspitz sign in psoriasis, intense pruritus in eczema).
Psoriasis: regular acanthosis, suprapapillary thinning, Munro microabscesses, parakeratosis, absent granular layer. Eczema: spongiosis (intercellular edema), irregular acanthosis, preserved granular layer. The Munro microabscess is specific to psoriasis.
Chronic plaque (most common, 80-90%), guttate (post-streptococcal), pustular (Von Zumbusch), erythrodermic (above 90% BSA), inverse (flexural), nail psoriasis, and psoriatic arthritis (asymmetric, DIP joints, dactylitis).
Psoriasis: topical steroids + vitamin D analogs, phototherapy (NB-UVB), systemic agents (methotrexate, cyclosporine), biologics (anti-TNF, anti-IL17/23). Eczema: emollients (cornerstone), topical steroids for flares, calcineurin inhibitors for face, antihistamines, dupilumab for severe atopic dermatitis.
For comprehensive dermatology MCQ practice, use the Medicine subject page — dermatology questions fall under Medicine in NEET PG. For another image-based walkthrough, see our ECG interpretation guide for STEMI.
This content is for educational purposes for NEET PG exam preparation. It is not a substitute for professional medical advice, diagnosis, or treatment. Clinical information has been reviewed by qualified medical professionals.
Start practicing dermatology MCQs today — the pattern recognition you build now is the recognition you will apply on exam day. Start with 15 free dermatology MCQs. Want unlimited AI-powered image MCQs with visual explanations? Explore NEETPGAI Pro.
Written by: NEETPGAI Editorial Team Reviewed by: NEETPGAI Medical Advisory Board Last reviewed: April 2026
This article is reviewed by qualified medical professionals for clinical accuracy and exam relevance. For corrections or updates, contact the editorial team.